I would say so if you're considering it. Say something along the lines of "I just want to let you know that I do have a stutter so in case you hear it, I'm not just nervous." Depending on how serious it is (as it's a volunteer program and not a job-job) and the vibe you get, you can make a joke about it. I know for me I'll sometimes bring it up and not stutter at all so I'll say "I just want to let you know that I do have a stutter... it sometimes comes up sometimes doesn't... it likes to be unpredictable"