Maybe those who wish to close their account could be asked before leaving whether they also wish their group to be closed. I mean if you're leaving the site you're not necessarily interested in something you won't be involved in anymore - in fact a lot of people start a group and then contribute to it very little thereafter. It doesn't have to list their name still but if threads by ex members don't get deleted i don't see why their groups have to either.

Maybe some sort option like this can be put in place? I'm sorry the group closed - is there another you can attend or perhaps you can start one yourself?