Quick answer
Make sure the host and every guest use the same current game branch. Recreate the lobby after anyone changes branch or updates the game. Up to six players can be selected, although the developers describe three players as the intended balance.
Connection checklist
- Close the game on every PC and let Steam finish pending updates.
- Open the game's Properties and confirm that every player uses the same Betas selection.
- Return to the normal branch first unless the developer currently instructs players to use a network test branch.
- Have the original host create a new lobby and resend every invitation.
- Test with three players before raising the lobby limit to separate connection trouble from high-player-count issues.
About the old network beta
The developer previously published a network-issues test branch as a temporary troubleshooting option. Because beta branches can become obsolete, use it only when it is still visible and the current official announcement recommends it.