I arrive at Tefilot on Friday evening early. Since I’ve already brought in Shabbat for myself, what am I to do during Minchah? I’ve been standing and responding during Kaddish, and standing during Kedushah. I don’t know whether I should respond during Kedushah. And I don’t know if it’s inappropriate to sit during the time that the others are reciting Shmoneh Esrei.

You can sit during the Amidah and Chazarat Hashatz and of course you can respond to Brachot and Kedusha. You can learn or say Tehilim (Mishna Brura 90; 58) but not talk otherwise (Tefilah Kehilchata 13; 35). You can't sit within 2 meters of someone that is in the middle of Tfilah unless you are in the middle of learning Torah. (Mishna Brura 102; 13)