There are a couple of ways. One is to get a quote from a bus company, and then organize everything yourself. Plan on your breakeven point at about 35 people (if the bus holds 50-55). For everyone over 35, you'll make money.
Your promote it like everything else ... get out and network, post flyers everywhere, advertise in local papers, etc.
Another way is to partner with a bus company, and sell on their behalf. You won't make as much money, but there's much less risk.