There are some things that i have to consider.

There is a fundamental interactive design problem of making a multi-way conversation with a shared map work seemlessy. There are other issues such as logistical problems that need to be thought about, however these are all very small extras. The fundamentals of the way the communication works will be what makes or breaks this application.  
So for now i am going to have to forget about some of the small logistical problems and focus on the actual interactive design problem.

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