The use of subgroups will partly depend upon the features of your platform. Subgroups can be a nearly-certain requirement on some platforms to allow further segmentation for marketing and analysis purposes.
We use groups for local branches of a national organisation. We use subgroups where they require private conversations e.g. amongst the leadership team. That’s because it is the easiest method to control access to information in certain categories.
We could have provided this in a slightly more unstructured way by encouraging the use of personal messages to multiple recipients.
If your platform already has personal/private messaging to multiple recipients then that is already possible for your users to take conversations private. If so, then you could explore your database to check on what is actually happening with such conversations before implementing a more obvious method to do the same. If the messages are private then you might only be able to see who the “group” members are but even that could be useful.