I recently presented a paper at CTS 2005 in St. Louis, USA on the topic of a mobile group's usage of mobile discussion list. We ran the study in Northern Australia and found out some interesting usability issues that groups encounter when engaged in search and rendezvousing tasks in natural environments. Those of you developing group SMS applications may find the results informative for your designs.
Have a look at the paper yourself here:
Axup, J., Viller, S., & Bidwell, N. (2005)Usability of a Mobile, Group Communication Prototype While Rendezvousingpdf
We are currently involved with some similar research involving backpacking groups using mobile technologies which has not yet been published.