It seems if the first player who enters the server gets in by connecting via the IP, it disables the invite option because there's no actual lobby and the invite option is to invite into the lobby, not the server.
- In client console at main menu enter "mm_dedicated_force_servers SERVERIP:PORT"
- Create a lobby of some sort
- Under lobby options pick "Best Dedicated Server Available"
- If versus mode wait for someone to join
- Start game (the mm_dedicated command will automatically make you connect to your server by default)
- May all need to execute sb_all_bot_game 1 so the server doesn't shutdown if someone doesn't fill the other team.
- Let server populate or invite friends
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