For me personally, in this situation members of a group feels more like something people have been assigned to, something they belong to - a status you're talking about in a more general sense.
Members in a group carries more of an immediate, dynamic feel - like the group is actually doing something together, actively working on something, or several people have physically formed a group by coming together. It feels more like you're describing a specific situation, rather than a category people have been assigned to, if that makes sense.
It's really a (subjective) tonal nuance - of and in both carry the same idea in this context, and are interchangeable outside of a few common phrasings (e.g. several people together are in a group, of a group works but probably sounds unusual to most people).
In is also a preposition for actual physical location, so in some contexts (where an organisation is also a place) it might be unclear which meaning you're going for:
I think that's partly why in feels more immediate in general, it carries that same sense of "the situation right here, right now", whereas of can sound more distant and abstract.