Some other things I think people tend to do together unconsciously:
-Smile. It is almost impossible to look annoyed around someone you like when they're smiling and doing the whole high-pitched, "Smile. Come on...you know you want to..."
-Share a similar style of humour. I find I do this with friends and family...my style adapts ever so slightly to better mimic the style of someone I'm talking to.
-Gang up on someone. If people mutually dislike someone, they will band together without even thinking about it.
-Be more likely to watch a scary movie while in a group.
-Be more likely to try something new if the group, together, insists someone do something with them.