code: def _link_user(self, userid):
          
          # check if we have a selected user from the search-list
          if userid:
              try:
                  self.context.setUser(userid)
                  self.add_status_message(
                      _("User linked to this Contact"), "info")
              except ValueError, e:
                  self.add_status_message(e, "error")
          else:
              self.add_status_message(
                  _("Please select a User from the list"), "info")
docs: Link an existing user to the current Contact