Wow, good question. I personally haven't heard of any. Sorry!
It's really interesting how when you begin to explore something new that related resources seem to find their way to your door. Just this morning I received a link to this blog about group texting applications: http://www.frogloop.com/care2blog/2011/4/25/group-texting-useful-or-hype.html and LaDonna Coy shared wifitti with me. I haven't had a chance to explore these tools...if folks have recommendations or are willing to share their experiences with any of the tools, I'm eager to learn from you!