Answer: Those who are plugged into a power outlet and can charge (or get some sort of credit) for the service.
Most of the MANET routing stacks provide for optional routing anyway. So, if you don't want to forward for someone else, don't.
There has been a lot of work done on MANETS. Just search almost anywhere for "mobile ad hoc networking". Wikipedia has a short article that looks like a good starting place for a beginner.