thibaultmol There are two ways to do this. The first is by using a plugin for private discussions: https://discuss.flarum.org/d/4762-friendsofflarum-by-bu-well-integrated-advanced-private-discussions
The second way is to show a specific tag only to a certain user group.
In the Tags extension page, there is a button labeled something similar to "Configure Scopes". By clicking there, you can restrict the desired tag to a specific user group.
I'm not sure what "configure scopes" means in the original language, my local language uses something similar