I think siblings play an important role here. More than a few times I remember going out and having no one to play with but my brother, only to be joined later on by other kids.

Moreover, I can only assume that it was through me that my brother got introduced to the "social scene" of the neighborhood. I was the older one, so when we came out to play, I was in charge of him, and that meant always inviting him to play and introducing him to all my friends.

Siblings are like friends you have no option but having (and playing with). Incredibly reliable, in that sense (: