Here are the rules for salatul jamaa

For the Ma'mum

The lines should be straight, with each person standing close to another – shoulder to shoulder. As long as there is a gap in any one line, that spot should be filled before making a new line, and it is Makruh for a person to stand alone. It is Mustahab that after the line:

قد قامت الصلاة

is read, the followers rise and get ready to start the Salat. It is Makruh for the followers to recite any of the supplications, TasbIh or TakbIr, in such a way that the Imam can hear them. When a Ma'mum wishes to join the Imam in Ruku'; but fears that the Imam may rise before he can join, he can make a polite appeal by saying:

يا الله or إن الله مع الصابرين

signaling to the Imam that he wishes to join. After the signal, he should not waste any time and join otherwise it might inconvenience the other participants. He should then pronounce the Takbiratul Ihram loudly so the Imam becomes aware that the appellant has joined.

The Qualifications of the Imam of Salat al-Jama'at
One who leads the prayers is called the Imam, and he must be:

1. Baligh (Adult) meaning that he has attained the age of responsibility and puberty; and for Men, it is one who has completed 15 lunar years, or has seen in himself the signs of puberty or adulthood.

2. Sane (' Aqil)

3. Shi'a Ithna 'Asherl (believes in the mastership of the 12 Imams)

4. 'Adil (Just) meaning that he does not commit the major or minor sins.

5. Legitimate birth.

6. Able to recite correctly.

A woman can lead the Salat only if all the followers are women; however, if all of the followers are men or there is a congregation of men and women, then the Imam must be a man.

How to Join Salat al-Jama'at
Make the niyyat (intention) that you are praying behind and following the Imam who is leading the Salat. It is not necessary to know his name, however, you should know that you are following that particular Imam. Your Salat will not be counted as Salat al-Jama'at if you do not make the intention of following the Imam – this is important.

The follower must say the Takbiratul Ihram (الله أكبر) after the Imam, so as to maintain his Salat al-Jama'at.

Listen to the Imam
In the first two Rak'at, th e Imam will read Surah al-Fatiha and one more complete Surah of the Holy Qur'an. If you are praying the salat of Fajr, Maghrib and 'Isha, then the Imam will be reciting these Surahs out loud, thus, you should keep silent, and listen to his recitation. In the salat of Zuhr and 'Asr, the Imam will be reciting in a quiet whisper and you will not recite anything out loud, however, it is Mustahab that you recite any dhikr of Allah such as:

(١) أستغفر الله (٢) الحمد لله (٣) سبحان الله (٤) الله أكبر (٥) اللهم صل على محمد وآل محمد

Recite Everything Else in Jama'at
Apart from the two Surahs which the Imam alone will recite, you must independently recite all the other things in the salat. You must pray the dhikr of Ruku, Sujud, Tashahhud, Salam, and all the Takbirs, etc.

So remember, that a Ma'mum is exempted from reading only the two Surahs in the first two Rak'at.

Follow the Imam
As was previously mentioned, the Ma'mum must say the Takbiratatul Ihram (the first (الله أكبر) after the Imam has said it. If you say it simultaneously with the Imam or before, then there will be no Jama'at for you.

Then, in every act, follow the Imam. When the Imam goes into Ruku' Sujud, or rises, do it either with him or after him; but do not precede him. This is important as far as the acts are concerned.

However, in the case of the recitations like the dhikr in Ruku', Sujud, Qunut, etc., you may precede the Imam, even the Salam to complete the salat.

If you forgetfully rise from Ruku' or Sujud before the Imam, you must return to the Ruku' or Sajdah position (as the case may be) provided that the Imam is still in that position.

When Can You Join Salat al-Jama'at?
You can join the salat at any stage before the Imam rises from Ruku'. After the Imam has risen from Ruku', you will not be able to join him until he stands for the next Rak'at.

You can not join the Imam while he is in Sajdah, nor can you join when the Imam stands up after completing the Ruku'.

Some common Questions
Q. Can I join when the Imam is reciting the second Surah?
A. Yes.

Q. Can I join when he is in Qunut?
A. Yes, you can join, and perform the Qunut as well.

Q. How do I join when the Imam is in Ruku'?
A. Make your intention, say the Takbiratul Ihram and go straight into Ruku'. This will be counted as your first Rak'at.

Q. How can I join when the Imam is in the third or fourth Rak'at?
A. In the third or fourth Rak'at, the Imam does not usually read Surah al-Fatiha nor is there a second Surah, he will usually recite Tasblhatul 'Arbah. Therefore, if you join him while he is standing, you must read at least Surah al-Fatiha – you cannot simply keep silent. Therefore, if you believe that you will be able to read at least Surah al-Fatiha before the Imam goes into Ruku' and rises, you may join while he is standing. However if you believe that al-Fatiha may take a longer time to read and that you may miss the Ruku' with the Imam, then do not join while the Imam is standing. In this case, join him when he goes into Ruku'. It is always advisable to join the salat al-Jama'at in Ruku' if the Imam is in the third of fourth Rak'at.

Q. If I joined salat al-Jama'at in the third Rak'at of the Imam, then what do I read in the next Rak'at which will be my second, but the Imam's fourth?
A. You will treat it as your second and read Surah al-Fatiha and another Surah, and do your best to reach the Imam while in Ruku'. If you find that there would not be enough time for the second Surah and Qunut, then Surah al-Fatiha alone will suffice.

Q. Should my Salat be the same as the Imam's?
A. No, you can pray Salat al-Maghrib, for example, behind an Imam who is leading salat al-'Isha, and vice versa.

Q. What happens if a young child is standing in one of the rows? Does our Salat become void?
A. No, as long as the child is a Mummayiz, meaning he can distinguish between good and bad, and as long as the people around him do not have knowledge if his salat has become void, their salat al-Jama'at is correct.

Late Joiners
It is Mustahab to be present in salat al-Jama'at from its very start. If fact, there is a great reward in being able to say the Takbiratul Ihram immediately after the Imam has said it, and before he commences the recitation of Surah al-Fatiha.

However, if due to some unavoidable circumstance, you reach late, then you will read as many Rak'at of Jama'at as available, and complete the balance on your own.

For example, if you reach salat al-'Asr when the Imam is in his second Rak'at, then you will join, and when the Imam is in his last Rak'at, you will change your salat to Furadah (single), and complete the remaining one Rak'at alone.

When it is your first Rak'at and the Imam's second, then according to Ihtiya!, after the two Sajdah, you must sit in a position, flexing your legs, keeping fingers and feet on the ground, and listen to the Tashahhud being recited by the Imam (this position is referred to as Tajafi).

Similarly, when it is the last Rak'at of the Imam but you still have the balance to complete, it is Mustahab that you flex your legs with your fingers and feet still on the ground and wait until the Imam has finished reciting the Salam and then stand up to complete the salat alone.

When a person arrives so late that he finds the Imam is in his last Tashahhud, he would still be able to obtain the reward of Jama'at. He should make the intention, say the Takbiratul Ihram and sit to join the Tashahhud. Then when the Imam has completed his salat with the Salam, he should rise to begin the first Rak'at and he does not have to make the intention or say the Takbir again.

Connections in Salat al-Jama'at
The first person standing directly behind the Imam in salat al- Jama'at is connected to the Imam and so are the people next to him. However, as the first line extend s, people standing further away from the Imam are connected sideways.

Connections must always be maintained in salat al-Jama'at, for if it breaks, then there will be no salat al-Jama'at. For example, if a person standing in the second, third or any subsequent line loses all connection with no one in front of him or by his sides, his salat al-Jama'at ceases to exist, and he must continue his salat by means of Furadah. However, if he has someone by his either side who is in turn properly connected with someone else in front, then his salat al-Jama'at will be valid.

If anyone is praying salat al- Qasr, he should try to avoid standing in the first line, since if he chooses to remain seated after two Rak'at, he will become an obstruction to the people connected to him, rendering their salat al-Jama'at invalid and void. However, if he immediately rises again to join salat al-Jama'at, then the connection will be resumed and no harm is done to the other believers around him.

If a traveler prays in the second or any subsequent line, no harm will be done to the believers standing next to him even if he remains seated after the completion of two Rak'at. This is because people in the subsequent lines normally have more than one source of contact and connection.

Emergencies in Salat al-Jama'at
A. If the Imam's salat becomes void due to any reason, the Ma'mum behind him may steadily step forward to lead, provided that he fulfills the qualifications for leading the salat. Other followers will change their intention to follow the new Imam.

If no one goes forward to continue the salat al-Jama'at, then the followers will convert their salat to Furadah and complete the salat on their own.
B. If someone faints or collapses during the salat al-Jama'at, then the people standing next to him may break their salat to attend to him, provided that there is no one else to help him from amongst those who have not joined the salat.

C. If the Salat of anyone in the first line becomes void, he must get out of the line immediately. He must neither continue nor sit down, since it will be a distraction, and may render the Salat al-Jama'at of the others following him in the same line as void. When he moves out, there will be a gap, and thus the other people next to him must gradually move sideways to reduce or fill in the gap.

D. If for any reason, the salat al-Jama'at ceases to exist, the Ma'mum must not break their salat, they must simply change the intention from Jama'at to Furadah and continue until completion.

E. If due to some reason, the Ma'mu m has to abandon the salat al- Jama'at, then he can do so and change to Furadah. However, one cannot join the Jama'at with an intention of later breaking it.

Salat al-Jama'at for the Muslim Sisters
Muslim women may join salat al-Jama'at, but they will keep their position behind the men. Although in salat al-Jama'at, no obstruction between the Imam and Ma'mum is permitted, the women are permitted to stand behind barriers such as a partition, wall, curtain, etc. and be part of the salat al-Jama'at. The women are also permitted to have their own salat al-Jama'at in which a lady Imam can lead – as long as there are only women in the Jama'at. However, she will not stand in front, but will position herself in the center of the first line, and those following her will form the first line by standing by her sides.

A Few Narrations Concerning Salat al-Jama'at
The Holy Prophet, peace be upon him and his family, said: "Verily, when one of Allah's servants establishes the salat in congregation and asks Him something which He does not grant him, Allah will be ashamed until He fulfills it."

Imam 'Ali ibn Musa al-Ritja, peace be upon him, said: "The excellence of the congregation salat compared to a solitary salat, is one Rak'at to two thousand Rak'at."

The Holy Prophet, peace be upon him and his family, said: "One prayer of a man in congregation is worthier than his forty years of prayers at home (alone)."

Imam Muhammad al-Baqir, peace be upon him, said: "The person who abandons the congregation (salat) without having an excuse, and only for unwillingness or in order to avoid attending the gathering of Muslims, has no salat (his salat will not be accepted)."

Once a blind man came to the Prophet of Allah, peace be upon him and his family, and said that there was not anyone to take him to the Masjid to attend the congregational salat with him (the Prophet), when he heard the call to prayer. The Holy Prophet, peace be upon him said, "Stretch a rope from your house up to the Masjid and attend the congregational salat."

As for your second question no during that time it’s not makrooh at all to be dressed in black