You can have as many GMs as you want in your game (just promote them on the Campaign Details page by hovering over their picture after they've joined the game). However, keep in mind that for example the "GM Opacity" setting for the Fog of War and probably a few other things will be pretty much useless to your group. Which is fine if it's more of a collaboration style game that's less focused on virtual maps like D&D is. If you don't want to make them full GMs, they can still draw/write on the map and you can set all your tokens to "Controled By: All" and they'll be abe to manipulate those as well. So there's sort of a "macro" way of doing it (make everyone GM) or a more controlled/micro way (giving them permissions on individual tokens, characters, etc.)
