Hey,
Welcome to GS!
We had the same thing, and from my research and interactions with the GS team, it isn't possible unfortunately...
+1 to @jvdc
We faced the same situation during our community merge.
Sort of a workaround in a way that seems to have helped us - we did send out 1 or 2 newsletters post merge to let our members know of the merge and also gently remind them about subscribing again.
I also had this issue. We did these few things to try to get the subscription numbers back up.
We brought this up and showed individuals during super user meetings.
We sent user group leaders/managers a special communication on the need to subscribe.
We let CSMs know to communicate during check ins.
We communicated in emails sent to users during system upgrades.
We let product stakeholders know during community meetings.
Hope this helps and I would be interesting in learning what others did!
Have you tried using Invitation Links in your messages? From the group view in Control, you can create a link that will instantly add a user to the group when they click it, without having to request access. This has helped us before since it’s a much faster workflow for users; it’s easy to include those in an email or message, and it’s much faster for the user.