It sounds to me that perhaps Groups on Discourse could be one of the features that is more easily surfaced through the Setup Wizard.
For example, the admin can be asked:
What groups would you like to create?
And after entering a list of names, associated categories will be created which are just visible to people in those groups.
And when people sign up for an account, they can also be asked if they would like to join certain groups, if they’re publicly available. Or they can request to join groups, if the groups are available for requests to join.