Rika is right, some collabs here at PC are not just restricted to staff. In fact some involve regular members and supporters as well, like the collab the Pokemon Clubs chatroom members had featuring Super Smash Bros. Brawl characters, and Rika stated she and some RP regulars were doing a Rugrats collab.
And just like staff, Tier 3 and higher Supporters can access a special forum, and since I'm Tier 5 I can actually view all the Supporter forums. The staff are just doing a mini-collab right now featuring Pokemon Crystal sprites, which I think is pretty clever.