Is it possible to merge two groups?
Using Jamroom
I don't think that will work with GDPR , your users need to "opt in" for any changes. You can't assume, nor ask for their agreement by default (e.g. "you agree to be moved to a different group later if we feel like it").
If you have zero activity, well that's a different problem. Merging inactive groups is a waste of time. Merging an inactive group into an active group is high risk, don't dilute your active group and kill it.
Imagining group activity in advance of it happening is a huge waste of time, a fool's errand. You just have to watch and learn.