Hello, America!
I'm from Germany (Bavaria) and we have two big Christian denominations: Catholic and the "Evangelische Kirche" (mainline protestants).
When there's a big event, there's an ecumenical service with priests from both sides. Also, the local Catholic church sometimes helps out the Protestant community (which is way smaller here) logistically and they organize events together.
The US seems to have about a billion denominations. Do they get along and do stuff together (like a potluck)? Which denominations don't like each other at all and who are "friends"?