There is no constitutional right to be a volunteer ambulance squad member. So, this is not a civil rights violation, per se. A violation of a clearly established constitutional right under color of law gives rise to a Section 1983 civil rights lawsuit. But you should see a lawyer in your locale to fully discuss your situation.
You might find my Legal Guide helpful "How to Choose A Lawyer For You"
You might find my Legal Guide helpful " What Do I Tell My Lawyer"
You may find help in my Legal Guide "What is a Civil Right?"
Good luck to you.
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This question would only involve a Civil Rights issue if the ambulance was a government organization and if the membership restriction was motivated by discrimination of some sort. There could also be an issue if the individual restricted was treated differently then others similarly situated.