Have you considered asking your members to say “Hello” in the chat when they join your monthly group meeting? This way you would know whether they’re here or not.
You’re suggesting that OGS should have a feature to allow you to know which user is currently looking at which chat room. Personally I would be strongly against a feature that would allow people to spy on my activities to the point of knowing which tab or which chat room I’m currently looking at. So I think it’s quite a good thing that OGS only says which users are online, and not which users are currently looking at what exactly.
(I also would hate being kicked out of a club just because I forgot to close by browser before leaving home and so I was shown as online on OGS and that inconvenienced someone else. But that’s just my personal opinion.)