Here's the thing about prom-night hotels: the room belongs to whoever signed the registration card, full stop. That person's ID, name, and credit card all need to match. If anything goes wrong — damage, a noise complaint, an extra incidentals charge — it lands on them alone. Pick the friend with a credit card in their own name, put the room on that card, and have everyone else Venmo their share before you even drive over. Splitting the bill at the desk does not work. The desk runs one card.
You probably want a step up from a roadside motel — Hyatt Place and Holiday Inn Express both run mid-scale 18+ properties in every major U.S. metro, and they're the easiest yes at the front desk. If you're trying to keep it cheap, Motel 6 is the chain that says yes most often anywhere in the country — 96.5% of their properties are 18+. La Quinta and Days Inn run somewhere in between.
What you should NOT book: the Ritz, the W, the JW Marriott, anything with 'full-service' Hilton in the name. Those default to 21+ nationwide and the manager almost never makes an exception for prom. Manhattan in particular is a 21+ minefield at the luxury tier — if you're doing prom in NYC, look at Pod, Sonder, and the boutique-budget brands instead. You'll save money and skip the rejection.
If you're picturing the whole crew piling into one big suite for getting-ready photos and afterparty energy, slow down. Most hotels cap a standard room at four people and a suite at six. The front desk will count heads when you walk in, and ten people in a one-bedroom is the call the floor manager makes. Book two adjoining rooms instead. In most cities it costs about the same and gives you double the space — way better than getting walked out at 11 p.m.
Card-on-room math, because this is where prom groups get burned. If a friend trashes the bathtub or sets off a smoke alarm at 1 a.m., the cardholder gets billed. Not split — the cardholder. So if it's your card on the room, you are basically signing for everyone in it. Collect the cash up front, in advance, before you sleep in the same room as people who have been drinking. (You shouldn't be drinking — federal law is 21 to drink anywhere in the U.S., including the room — but you know what you know.)
Mid-sized cities are quietly the best move. Chicago, Atlanta, and Boston all have enough 18+ inventory that prom weekend doesn't blow up the rates the way it does in Manhattan or LA. Book three weeks ahead and you can land a decent mid-scale room in the $130–180 range. Atlanta in particular has the deepest 18+ pool in the Southeast outside Florida, mostly at the mid-tier chains — exactly what a prom group splitting a room is shopping for.