Is there a forum that specializes in getting players together in custom lobbies? kind of like the classified section in Bungie’s old site for the same thing?
You’re better off trying other websites for customs. Forgehub, for example, usually holds customs to test out forge maps. Hopefully someone else can provide more websites than I can, but I do know there are sites dedicated to customs and even certain gametypes (Infection, Racing, Grifball).
You could try your luck in the recruitment section of these forums as well.