mik5
You should answer Amein to all Brachos as well as answer to Kedusha (Nakdishach/nekadeish) as well as to Kaddish.
If you are learning and are in another room but can hear the minyan (even other side of mechitzah) you dont have to answer. Otherwise you should answer to everything as well as say Aleinu. Tachanun is not required, if you didnt say it you may say it with this minyan.
Did I miss anything?