Considering that I'm in a 2 dentist group practice, well you'd be correct if you guessed that my business partner does any restorative work that i've needed. My business partner and I also have 1 other dentist who has a solo practice in the same town as ours who comes to see one of us when he needs some restorative work (we don't charge him, and if he needs a crown, he has the lab he uses fabricate the crown and pays for his own lab work).
If treatment needing a specialist is needed, well then I just go to the same specialist as I send my patients too. I've never been charged for work in my own mouth by a specialist whom I refer too, since as they put it, "it's just a cost of doing business" Likewise the same goes if a specialist comes to my office for work, they don't get billed. Atleast in my area, we also do a good job at taking care of staff, with atmost nominal charges for labwork being passed on to them. That's how things tend to happen within the dental community in my area when a colleague needs some work done