I think the very best first sentence is "Hi, I am (name). I just want you to know that I have a stutter. if it happens, just wait and let me finish. That's all." Self-disclosure is so powerful. It might seem extremely frightening and very personal but I can't tell you how this relaxes both yourself and everyone else in the group. Most people don't know someone who stutters so they don't know how to respond when it happens. "Do I look away to make him feel more comfortable? Should I finish his sentence?" There will be a lot of chatter in their heads. If you pre-empt this, you show that it's not a big deal, no taboo, just something you do, you invite them to ask questions or not but mainly you can then all focus on what really matters: the musical.