@cjs56 and @Robin_Dickinson, I certainly see this as a valid DPC concern about any third party payer, whether it be the employer, a friend, an insurance company or Medicaid/Medicare. I have no definitive answers, but maybe a few helpful safeguards. (BTW, I have 58 employees from 8 different employers, with the largest two employers having 21 and 18 employees. I have 374 privately contracted patients that make up the rest of my panel.)
1. Contract with large employers (my definition of large is >50) with some trepidation. As you mentioned, losing one large employer can have dramatic repercussions, as I suspect most employees would not sign up for our plan on their own, if the company dropped their contract with us. (Not that our plan is expensive. It's not, but that our community is indigent).
2. Advise the HR person or owner up front that you are the patient's physician, not the company's doctor. You appreciate their willingness to offer a payroll deduction or pay outright for their employee's membership fee, but when push comes to shove, you will advocate for what you understand to be the best care you can offer to the patient, payer not withstanding.
3. As Robin indicated, develop relationships of trust and integrity, first with the patients/employees and then also with the employer.
4. Recognize that member-by-member growth is slower, but also more stable. People are more inclined to appreciate the value of your DPC service when they are contributing to the cost. When someone else pays for the care you offer them, they are less likely to appreciate it and more inclined to feel "entitled" to their own whims or perceptions of good care.
5. It is probably worth involving an attorney in developing a contract with a large group. For instance, I have courted a local soft drink company with well over 300 employees and would need to hire another provider if they signed up. For that reason, I would want to have a guaranteed contract length and/or large early termination penalty to help compensate for all the trouble in having to release a provider I could no longer afford. Plus, with a longer contract period, it is more likely that the new provider will have established a significant enough patient base to continue to work even if the large employer contract ended.
Not sure if any of this helps, but some thoughts that came to mind at first blush.