how can i restrict messaging to members of the same subscription?

Hi WPMU,

My plan is to organize location based meetings, members can sign up to these location based subscriptions. Before and after the meeting they can exchange information with wordpress messages, but only with the members that signed up to the same subscription. So members that signed up for Alberta Meetings can not send messages to Boston or Chicago members.
I would like to restrict messages to members of the same subscription. How can i do that?