From what I understand of the portrayal board, it's basically like a set of tabletop RPGs running simultaneously. When you find new a game you want to join based on the 'sign up thread'-it usually has that in the title, or "OCC" (the person acting essentially as GM will usually create this before beginning the game so you should have some advance notice) which acts as the character creation section (usually with instructions and guides as how to sort those aspects out and what information you need to provide for your character). Then you just wait for the game to start.
I'm afraid I'm not sure on the protocol for joining games already in progress as I'm not active on that particular board.
I have experience in the RolePlay scene of sites like msparp, so I know how that works.
but I'd like information on how to join, indeed.