Yes, yes it is. After the prospective GM has joined the game, on the main game page (before launching the game) you should be able to see a list of the players signed up. You can right click on the player's icon and you'll get the options to kick them or promote them to GM. Not aware of any limit to the number of GMs. If they are going to be trading off being GM and a player, they (and you) can hit the Rejoin As Player button in the Options next to the Exit Game button.
