Ok, can you confirm what you mean by ‘our platform’?
Are you referring to your Discourse installation, or something else?
Providing the group that’s created is visible to all users, anyone can apply to join, group owners can then approve or decline those requests. The group setting is called Allow users to send membership requests to group owners
As long as the category is secured to that group and any user who applies is vetted the audience to those discussion topics can be controlled.
There may be other options here, but we would need more information on how you’re using Discourse today and any other technology within your environment that these users might interact with.