Our singers' circle has a mix of people who sing unaccompanied and those who accompany themselves. An unwritten rule is that the musicians don't play along unless invited. An a capella singer can be thrown off by accompaniment, particularly when it tends to force them into a strait jacket of rhythm or key.

As someone who does both on occasion, I tend to announce the key if I'm inviting other people to play along. I usually don't do so if it's a complicated chord pattern (or if I want to show off my own playing).

Occasionally someone doen't know the rule, either a newcomer or an old-timer who's clueless (yes, we have them!). This can be awkward. For my own part, I'm not confrontational, but try to set an example. If I'm capable of playing along, and choose not to, you'd think the message would get through. But it doesn't always work. Sometimes there's a tactful way of making the point, but I can't think of one just now.