I’m sure they meant well, but, yeah, that’s rude. I would say it’s fine to ask him, privately, how he prefers to handle introductions. My husband stutters and in situations like that he would prefer, if asked, that the host or the person next to him introduce him. But, if he started saying his name, he’d rather not have people try to finish it for him. I’ve had people ask me the best way to respond when they are talking to my husband and I usually tell them, “Just don’t be weird.” Focus on what the person is saying, and just act like you would with anyone else. Don’t make weird faces or comments. Depending on how the guy stutters, sometimes it can be hard to tell if a person who is blocking (trying to talk but not making any sound) is trying to start speaking or not done speaking. So if you are leading the group you can just be conscious of giving this guy a few moments after he seems to be done to make sure he's actually done.