Parties are an old feature of Res. I'm not sure exactly what they entailed as they were already gone when I joined, but it was something to do with clubs/groups of users. There aren't any plans to bring them back in the near future. The soonest they'd come back would probably be in v4, but they may not even make a return then. We'll just have to wait and see ^^
(By the way you don't need to make a new thread for each question you have :D I usually subscribe to any thread I post on so if you post again on the same thread I'll get a notification about it~)