I'll just examplify:
Playing Neverwinter Nights online on an RP-required server I grouped up with some fellow dwarves. We hung out when playing, doing dwarf stuff; mining, singing and drinking ale. We formed a guild (even though that wasn't a function in the game) and recruited more dwarves. Our guild acted as a neutral faction beteen good and evil groups. 1½ years of it and we had 80 members making me believe the server was bigger than it was - a DM told me after the server shut down that about 2/3 of the players on the server had alts in the our guild. Learning to know all those chars and all those players has been one of the best RP experiences in my life. Almost 20 years ago now and some of the people I met through there are still my friends today.