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 like Tripp Lite, Leviton, Eaton or at least one of the better APC models. You don't want a cheap, small, 14 gauge extension cord connected to anything that you're going to have that many systems plugged into so a good ten to twenty foot 10 or 12 gauge cord should be essential along with a very high quality power strip. Don't make the mistake of thinking companies like Monster, Belkin or Amazon make high quality power strips either, because they don't.