If you're thinking specifically of doctors, I can offer you a patient's perspective on their language.
In thinking back to some recent eye treatment, and how the various practitioners referred to each other, there was definitely something of a range, i.e.
- another doctor in the same large practice would be a colleague, or spoken of as a specialist, e.g. "before you leave, book an appointment with one of our opthalmologists".
- a doctor with an independent practice, but well known, would be spoken of as a referral, e.g. "one of the people up front can give you some referrals".
- a doctor outside the group would simply be a name, e.g. "I'm sure we can find you some names."
In my experience with U.S. doctors, they are careful to avoid "endorsing" or "recommending" a fellow practitioner, unless they are linked in some way, e.g. through common employment at a large clinic. It's related to their concept of liability and the possibility of being sued by a patient or harassed by an insurance company.
In larger organizations like the Summit Medical Group, what you describe would probably be called a service team.
For something looser, such as doctors linked by common membership in a hospital billing system, you could maybe use affiliates, or clinical affiliates as a general term. However, the terminology is increasingly set by corporate medicine, such as RWJBarnabas.
It seemed to me that the doctors themselves didn't like this very much, which might affect how you use the corporate lexicon in what you are writing.