Someone need to leave the game? Never fear, that doesn’t mean you all have to quit.
You can ask other users if they want to jump in and replace the leaving player. If you find someone they must click the leaving user in the game followed by “Offer to replace this player”:
If you cannot find someone willing to replace the leaver, a player can be replaced with a controllable vassal house. All players of the game can click the leaver and launch a vote to replace them by a vassal:
See Replacing a Player by a vassal in-game for more details.