Creating a group practice
When I shared my office with other MTs (me + 2 MTs in a 3 room office,) I leased the office solely and subletted the other 2 rooms. The rental agreements ( + any cross-marketing agreements) were in writing. Everything was clearly detailed in the agreements, down to who was responsible for cleaning what areas of the office.
It worked out really well...I was held to a lease, but I gave the other MTs month-to-month tenancy agreements; that way no one was "stuck" if things weren't working out. There were a couple of occasions over the years where I had to pay for a room that was empty (but that was usually short-lived) but I'd rather pay for an empty room than to be stuck with someone in my office whose behavior was either generally inappropriate or at least not fitting with the overall harmony of the office.
The best advice I can give you is to make sure everything is openly discussed and the results of that discussion is put in writing for everyone to have. I can almost guarantee that if any part of your agreement is undocumented, it will be THAT part that will become the center of a "he said/she said" disagreement later on.