I am trying to think of a way to maintain game balance. BUT for someone new to join and have fun and to have a chance to win is hard. I would like to make it so that someone could join a game, late in the game, and still have a chance to win. I don't think it right to make a person wait for a game to be reset all the time for them to join for it may be weeks or months before a game will be restarted.
New players get an amount of money that somewhat corresponds to the strengths of the other players.
If you have a lot of old, large empires, then the new players start with more, so that they have more of a chance. A new player will start with a lot of money to buy cheap military units, so that he has a chance. It also gets harder to play as time goes on, so any one player can't stay at the top forever. If you make it easier to run a small empire than a large empire, then it won't be very easy to stay at the top, unless you're really good. It gives your new players a chance immediately, rather than having to survive for some amount of time before they can compete against the others, or only competing with people that join at the same time as them.