I've been reading through the online game threads on the welcome to the shadows board, and I can't quite understand how they work. I've played online games before, but only using chat programs, not forums, and always with complete character sheets. How do those forum games get started? Does the GM PM people to invite them in, or just start the thread with a cue like "The Johnson waited for the runners to arrive" and people jump in? And since I don't see any skill levels or dice rolls in some of them, how are the mechanics worked out? Does the GM just make up a more or less expected average result for every action?

Most of them start with the GM posting an OOC thread stating the type of game, chargen restrictions, and whatever other criteria he or she deems important. People then respond with character ideas and such, which the GM reviews and then chooses the players.

Most of the games have both an IC and OOC thread, with the mechanics and out-of-character chatter going in the thread that is labeled OOC.

There are a couple that have started that were 'invitation-only' games, but when this is the case it is usually mentioned in the first post.

All this and more at www.online-roleplaying.com, in case SR isn't what you're after on a PBB.

Most of the threads on Welcome to the Shadows are used for In-Character (IC) and Out-of-Character (OOC) participation in a run, and you shouldn't post to one of those unless you get in while the GM is still looking for players. While most runs there never finish, a good one called "Exile's Hope" did. A good way to read Exile's Hope is to open up the OOC and IC threads in two different web browsers, and read forward hour-by-hour across the two threads from the beginning. Dice rolling is done, and gets posted in the OOC thread.
Exiles Hope OOC
Exiles Hope IC

Other threads on "Welcome to the Shadows" are players posting "In Character" without a GM. A popular one is Night at Reno's." Anyone can join a thread like this with an In-Character post, without needing an invitation.