I live in Sweden and here at work we have both females and males working and it is normal that they sit together at lunch to eat and talk. Is it permissible for me to also sit with them, as long as such a gathering is devoid of sins such as Alcohol and music and I am sure I won't look with a illicit look at those women without hijab?

Are jokes that come up naturally during discussions fine as long as they are not about something sexual/bad topic and don't lead to any other sin? Considering it would be disrespectful to just ignore or be quiet. Or is any kind of joke between opposite genders haram in itself according to Sayyed Sistani? If yes, is it as a precaution (ihtiyat) and can one refer to the next most knowledgeable marja in this issue?

1- If the setting is professional and nothing immodest occurs then yes it would be permissible.

2- Ayatollah Sistani states that any joking with the opposite gender is haram. If you are caught in a situation like that, be creative and find an appropriate balance. Don’t disrespect them, but at the same time avoid joking with them. If they naturally mention a joke, just handle it professionally by saying something kind and respectful, without carrying on with the joke and then giving your own joke.