I tell everyone, when it's relevant to some (also light) conversation. With many friends I discuss problems we have or just problems we hypothetically have (some are less open about it) if someone present is unaware of some of the problems one might have. It makes it easier for them as well, because we can discuss, say, anxiety or panic attacks or depression as if only I have experienced them (while maybe all others have, in some cases, but just don't know it of each other). So like a deflector of sorts.

I always talk about symptoms, say mania or depression when relevant, but never say something like: "I have bipolar, so..." or "I was diagnosed with schizoaffective disorder" (unless specifically asked for my diagnosis). I never ever talk about it as an illness.

I'd suggest talking about symptoms (first), your problems, saying you need help from others to deal with those problems, but not in medical terms: you may describe mania or depression, but saying something is due to mania is meaningless if people don't know what mania is, for example. Or just say nothing.

But being open about it can be liberating. Just don't start or even mention names for things, but just the things.