When you get to the con, the registrar has put you in a family group. Normally you have a family group leader who is on DYSC if it is a DYSC- run con or is in the host youth group. The purpose of a family group is to get to know people and to have a place to touch base every once in a while since conferences are often very large. Your family group is put in charge of either preparing or cleaning up after a meal, so the work is divided somewhat evenly among conferees. Your family group is also your team if the Saturday night activity is a game that requires teams.
Coffee house is simply a talent show.
Conferees can sign up to perform at coffee house on a big piece of paper that is posted somewhere visible during the con. Performances at coffee house can be skits, musical stuff, poetry reading, etc. It normally lasts a couple hours and is a great show.