The SMS-based 3jam service lets you send a text to one number, which then gets multicast to everyone in that “group”. They’re free to use right now, and normal text messages still apply. But it’s one text per group, despite however many members you have in the group.
There’s no obvious money-making model here right now, though there’s a great money-losing one. But 3jam looks to partner with wireless carriers and get a cut of all text message fees.