Experience Cloud Users are unable to mention internal Users in Chatter.

Set the ‘Default External Access’ of the User object to 'Public Read Only.’ This is the only way to allow Users to @mention internal Users from the Site.
 

Set the ‘Default External Access’ of the User object to 'Public Read Only’

In Salesforce Classic

1. Click Setup |  Security Controls | Sharing Settings.
2. Click Edit.
3. Change the ‘Default External Access’ of the User object to Public Read Only.
4. Click Save.
    

In Lightning Experience

1. Click the gear icon.
2. Click Setup |  Security  | Sharing Settings.
3. Click Edit.
4. Change the ‘Default External Access’ of the User object to Public Read Only.
5. Click Save.

In cases where setting the User object 'Default External Access' to 'Private' is required, you can enable the 'See other members of this Site' in the Experience Workspace. This will let Users see each other in the Site regardless of the external organization-wide defaults.
 

Enable 'See other members of this site'

In Salesforce Classic

1. Click Setup |  Customize | Digital Experiences | All Sites
2. Click Workspaces | Administration | Preferences
3. Enable the See other members of this site.
4. Click Save.

In Lightning Experience

1. Click the gear icon.
2. Click Setup |  Feature Settings | Digital Experiences | All Sites
3. Click Workspaces | Administration | Preferences
4. Enable the See other members of this site.
5. Click Save.

To @mention using a Site member's nickname, Enable nickname display