Trc4 if you are referring to the bundled Mentions extension from Flarum, in the admin panel under the Mentions extension page you will find the Permissions section with a single permission, that's the permission that controls who is allowed to mention groups.
If you are thinking of a different extension or feature, can you share more information? If you don't remember which extension it is you can look through the list in your admin panel. You can share your php flarum info output with us so we can see the list of extensions and their versions.
