I agree with asking. My husband stutters, and he does appreciate people being told in advance, but how it’s done matters. He would definitely prefer that it be disclosed in private, rather than as something announced to the group at the start of a meeting. He was a GA to a professor in grad school who announced his stutter to the class when he introduced my husband to them, and he found that really embarrassing. But he may also not want you to say anything, or might want to disclose it himself. I think a good rule of thumb is that adults who stutter have dealt with lots of situations like this and know how they want them handled, so it’s always best to just ask.