"Co-worker" equivalent for "volunteer"?

Question: What is a term that can be used to describe someone who volunteers with you? Is "co-volunteer" an accepted term?

Elaboration: This question was brought to my attention when writing down references for a job application. The field, "relationship to reference," made me realize that I didn't know of and couldn't find a term that is similar to a "worker's co-worker." Since I couldn't find validation for the term "co-volunteer," I needed to ask this question.

Note: If this is a duplicate or posted in the wrong place, please point me in the right direction. I was unable to find this anywhere.

The phrase I would recommend is "fellow volunteer". You can't go wrong with it; it's two well-established words. Here's an example:

Getting to Know Your Fellow Volunteers

Alternatively, there are some people who use "covolunteer" or "co-volunteer" (h/t jsw29):

Both Carly and her covolunteer from Virginia are kitchen-challenged—even though the covolunteer has already worked there for seven weeks. — Ecuadorothy

Like Mimi's covolunteer at the homeless shelter, Leticia had perceived CAPS and the Northtowners as racially divisive — Us Versus Them: Race, Crime, and Gentrification in Chicago