Generally in north America, all you have to do is tell the bar manager or the chief bartender that the next round is on you. They will do the rest.
Depending on the type of establishment your are at, they might just ring a bell and shout it out, or they will just start pouring a round of what everybody is already holding in their hands and start sending it to the tables. Normally, the staff willl let everyone know you are buying a round for them.
For budgeting, plan on spending US$5-7 per head, and please do not forget to tip the bartenders and waitresses. As unfortunately, most benefactors of your kindness will not tip on the free drinks they get.