I co-own a Gammill Classic Plus long arm with 3 of my sisters (oops with 2 sisters, one has passed away). We split the cost of the machine equally and we each buy thread and patterns etc on our own but everyone is allowed to use whatever ... we do not have a written agreement.
We do get together once a week for quilting so we know what each other is working on and when each will need the machine. We try to take turns but if one of us needs the machine out of turn we just ask if we can use it ... we haven't had any problems in 5 years.