Chips, salsa and beer. Maybe some sandwiches. Ok, just kidding. Honestly about the only thing you really need is a router or switch with enough ports for everybody who won't have a wireless adapter and a wireless signal from your router for the rest of them. If everybody has wireless capability, and you already have a wireless router, then you probably need nothing.
I can't imagine you're going to get very good speeds with 10-15 people on one internet line though unless you have a very high bandwidth fiber connection.
Are these going to all be desktop systems, or laptops? You might also want a way for everybody to plug their devices in as well so probably a decent 10 gauge extension cord and a high quality power strip made by somebody...