iPhone: Fast Society is a group messaging, calling, and geo-location tool that helps you stay in touch with groups of friends temporarily while attending concerts, heading out on the town, or at other events.
Although the group starter needs the have Fast Society installed on their phone (and ideally, the other users would have it too) the system works well enough in mixed groups. One person starts the Fast Society group which expires within 3 hours to 3 days based on the creator’s preference. They add people from their contact list to the group and those people accept the invitation via SMS. Once everyone is in the group they can group text, geo-locate each other, and do group conference calls.
For another group messaging solution, check out previously reviewed GroupMe.