It sounds like he has a group of friends and I assume you have a few already? There's a difference between "friend" and "friendly." He waved, you do know one another, waving, stopping to chat a moment, etc. is going to happen! Who knows if he or you are interested in anything more yet? You have to go through the "getting to know" stage like any other friendship, you all knew one another from a different type of relationship. He doesn't know you out in the "world" and you probably don't know that much personal about him, normal friend knowledge, either. It's not a black and white, all or nothing thing where you either are or are not friends, it's a growth thing like anything else. You check out whether each of you wants to know/be more than acquaintances and go on from there.