People who check in but also Facebook friends who are slurped in from the API.
A table of who is friends with who. There should only be one entry for each friendship.
For example, for user_id and for friend they could be the flipped but represent the same relationship. This should be avoided by the code.
Each mutual friendship record is one friend that a user shares with one other person at the party.
user_at_party is the main user at the party in question
user_at_party_2 is the other user at the party who has a mutual friend with user_at_party
mutual_friend is the friend that user_at_party shares with user_at_party_2 but that person may not be at the party
Again, if you flipped user_at_party and user_at_party_2, you'd have the same relationship with two entries which would be bad. The code should avoid that.