Any promotion I do would have to be based around the concept of also inviting others, as a chat service is obviously only useful with friends. Maybe people could become eligible after they invited x number of friends? I'll think about it, thanks for the input.
As for the usability factor, I do think it is certainly there. I have often heard complaints that while texts can be sent to multiple people, replies only go to the sender, preventing group chat or collaboration. My service, in essence, provides that reply-all button. If you want to check it out, follow the directions at http://slide.fm/ (I direct traffic from advertising to a different, more teen-friendly landing page, just FYI).