Adding Expansion Players

If you think there'll be other players joining later in the game, add an Expansion Player for each missing person. Stars! will assign a "housekeeper" AI to run things for those players until they actually join the action. This AI will keep planets and fleets active, making sure the production queues are busy, etc. It does not develop any strategy.

When the player joins, right click on the diamond next to their name in the host dialog and change the type to Human Controlled. Then, give the latecomer a break by asking the other players to leave them alone for N years.