GroupMe
One of the first and still the best. Allows IM, SMS, photos, checkins, conference calls and more. A bit of an underdog but an app with staying power.
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Fast Society
Intended to be a spontaneous group comm app. Offers many features but I still prefer GroupMe
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Messenger
The latest offering from Facebook. Allows you to message friends and contacts, share photos, and location. Great for checking FB messages without opening Facebook.
Google+ – interests, communities, discovery
They have introduced Huddles. Basic group IM but one to watch since it is tied to Google+
Skype for iPhone
Extend group conversations to your mobile phone, easy IM, and video calls. Not laser focused on group communication but too large of a network to be ignored
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