If you want to join Jamat namaz and you are sure that Imam Jamat is in either 1st or 2nd Rakat then you have to just say Takbeer (Allahu Akber) and join the namaz but if the namaz is zohr or Asr where imam Jamat recites the surah silently and it’s not clear for you whether he is in 1st or 2nd or 3rd then you have to wait until he (she) goes to Ruku then you join but if it’s 1st or 2nd Rakat and after finishing Surah Hamd if he says loudly Bismillah and going to recite another surah then you can join at that time. Even you can join if he/she recited Qunoot.
Only if it’s 3rd or 4th Rakat then you must join in Ruku unless you are sure that you can finish at least Sura Hamd while he/she recited Tasbeehat e Arab then you can join in standing position too.