I've little suggestion.
It would be nice to divide player's slots on public server to two groups:
- PUBLIC — Anyone can connect without password or with password for public slots. Players connected without password or with password for public slots would occupy public slots only.
- PRIVATE — Only those who know private password can connect. Players who connect using private password would occupy private slots only (not public).
Example: admin would enable four public slots and two private slots. He would setup password for private slots only. So such server could be occupied by four (random) players who don't need to know any password and by two players who know private password. These private slots could be used by admin of server to connect or by trusted players.