Yeah, to play a scheduled-by-Gen Con game, you need tickets. You can register for a particular game (at this point, registration would have to be done on-site), or buy generic tickets (also on-site) and use the generic tickets to get in on a game where there are empty seats. However, depending on what games & times you like, you may find that most of the games are already full. IME, most of the games I wanted to play filled up within hours (if not minutes!) of registration opening in May.
You can show up with generics & try to get in to a game, though -- sometimes people don't show up (early morning slots on Saturday or Sunday might be more prone to this
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
), or a GM might take an extra person.
Or find a pickup or informal (i.e., outside of Gen Con's official schedule) game. Check ENWorld's Gen Con forum, and check Circvs Maximvs's con forum (Hypogeum? something like that, but I don't have CM access from work to check); there are usually a bunch listed there, though they can fill up, too. Plus, you get to game with people you at least might know from the boards. RPG.net also has their own get together's, I believe, and probably other online communities, too.
You can also wander around open gaming areas (I think the Con has space set aside, and the Hyatt's -- I hope that's the hotel! -- a common area, too) and look for a game. Or start one.
(My suggestion: get in a game with HyperSmurf, if he's at this year's con; I was in an M&M game Piratecat ran last year, and the Smurf was hilarity personified. The best games I played last year were mostly ENWorld/CM games.)
Oh, and if you like
old original
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
school gaming, Ben Robbins & co. are trying to get
Major Wesely to run the Braunstein game again this year, as an ad hoc thing. Coordination at
Ars Ludi blog.