I just invited 8 friends out to dinner for my 21st birthday. Am I supposed to foot the whole bill?

I don't get out much. I've got a really strong inclination to stay in, but I figured I would actually end up pretty bummed out if I celebrated my 21st alone, so I just invited some people out. Should I go prepared to pay for everyone? How does this whole thing work? I went to my friend's 21st birthday at a restaurant and we all threw in credit cards and evenly split the bill, but I'm not really even sure how that was negotiated. It seemed like at some point everyone just kind of did it, and I followed suit.

I'm able to pay the whole bill, should that be necessary (although I'd rather not, especially if most of us are ordering drinks). How should I handle this? What should I expect?