Ex. Your mutual friends sharing inside jokes and then you all share the inside joke, even if the third party was not present when the said joke was made.
Friend: Hey this is my friend Bob
Friend of a Friend: Oh your Bob? I've heard so much about you!!!! (Super excited, and might even share an inside joke that she heard from your mutual friend)
Bob: It's great to finally meet you too, We are like Best Friends by Association (BFBA)