I am sorry. It’s a dilemma. I’d proceed with caution and try to find out what kind of group is it and what’s his goal in joining such a group. Is it for dating and finding intimate partners or is it for activities he enjoys and it’s just happens to be singles group? I have a married friend who belongs to singles biking and hiking group. Her husband wouldn’t bike and hike so the only good hiking group is singles group.

I remember in the past he had a problem with some of your activities. Talk to him first. Of course he might lie but ask him to show you what messages are exchanged in the group. It might be innocent

Also when you think of it you’ve had feelings for other people in the course of the marriage and I recall you reported you actually fell in love with one of your submissive or something. Your husband didn’t leave you but he requested you stop it or something like that. I know you didn't consider it cheating but he likely doesn’t consider joining singles groups cheating either (and technically just joining a group isn’t probably cheating).

So is the issue him being secretive and hiding that he is married or is it him joining singles group? Bottom line talk to him first and see what he has to say