How it works: You send your text messages to 3jam (since the service isn't launched, I can't tell you the SMS code). On your behalf, 3jam forwards the messages to your indicated recipients. When one replies to a message, 3jam again forwards that to everybody in the conversation.
I can see this being very handy to help groups organize places to meet. It's better than e-mail (who checks their e-mail all the time on their phone?), and it's better than mobile-based IM, because your buddies don't have to be logged on. And it's certainly better than trying to coordinate several different people via several different SMS conversation threads or phone calls.
Could it be abused? Absolutely: It could lead to a plague of cc's, like we have on e-mail. Hopefully this won't happen. Hopefully 3jam users will understand the value of a good message--and the social costs of annoying people with pointless copies.