Hmm, not sure exactly what you ran into, but the process goes something like this:
1) In game, hit escape.
2) On escape menu, click manage players.
3) On manage players menu, set one of the disabled slots to active (and set the role to whatever you want that person to be), and click save.
4) When the game comes back up and tries to resume simulation it will pause and say "Waiting For Players".
5) Have the person join.
6) Game proceeds.
If that doesn't work, try setting the name of the player in the slot you're opening to the profile name of the person joining; but in general if the slot name is the default "???" it should let anyone in.
