It is customary that when you invite friends or guests to a party you are hosting, you pick up the tab. However, there really is no hard-set rule against asking your friends to contribute to or split the bill when you go out to celebrate your 18th birthday. Just make sure to inform them ahead of time that you still can't afford to pay for all of them. Moreover, you also need to make sure your friends can afford whatever it is you are planning because they might also be in the same financial boat as you. For example, you cannot ask your friends to pay for their own meal at a super high-end restaurant when the cost of one meal alone can already pay for the entire group at a much cheaper place. In situations when you can't afford to shoulder all the expenses, say for a meal at a restaurant, friends are usually sympathetic and understanding. Besides, we are talking about your closest friends, and these are people who wouldn't mind spending a little as long as they could celebrate your birthday with you and see you happy.