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Organizers of large events often have need to keep various people abreast of action 'on the ground' as it were.
While the use of walkietalkies is common in professionally staffed events, there are places where the use of 2 way radios are restricted through the need for licensing or cost. Cell phones, pcs, pdas using sms, web, mobile web, instant message, or other messaging systems are the perfect solution to this.
I often attend events where various participants in different locations need to update each other on what's happening at any of a number of given locations. These updates would ideally be sent in to a group based system and re-distributed to other members of that group.
Why not use twitter? In a nutshell, privacy. It'd be nice to have this functionality in as private a 'space' as possible.