The "Dress up" bit is a pretty specific part of the RPG scene, more commonly known as "LARPing" (Live Action Role Playing) It was something I did briefly (usually at conventions) but I never really got into it that much.
And I embrace my inner geek (actually, it's not even inner, it's very much outer
)
A roleplaying group can consist of a group as small as 2 people (Gamesmaster and asingle player) or up to around 7 (1 GM, 6 players). It depends on the game you want to play. Games like James Bond are good with just 2, D&D is best at 5 these days. It really depends on the game. Check out that shop sometime (it's one I randomly found in Delaware), you might find a new hobby