From code side, Filters out portal users or external users then Link the user to their corresponding record (because Odoo stores partner info for both users and contacts).

class ResPartner(models.Model): 
​ _inherit = 'res.partner' 

  ​def _message_partner_allowed(self): 
  ​""" Restrict @mention to only internal users (employees) """ 
  ​ ​internal_users = self.env['res.users'].search([('share', '=', False)]) 
  ​ ​return internal_users.mapped('partner_id')