You are automatically made the GM of any new campaign you make, but under "Manage Members" section, any player can be "promoted" to GM at any time. Please note that all campaigns can only have one GM at a time, though.
Every campaign has to have a GM named, though, so start as the GM and join the players as normal. When you're ready to promote, go ahead and it will switch you as a player in the campaign.
HeavensAgent, I don't think that is possible without intervention of the site admins (and I don't know what their policy on this is). The current GM has to be the one to promote someone else to GM status (which can now be shared). Keep in mind that players can do most of what a GM can do, the main exceptions being (a) reading GM-only sections of pages and (b) inviting/ejecting players to/from the campaign. There may be others, like the ability to change the campaign banner or upload a map.
Are you confronted with such a situation now, or were you just musing?
I'm just musing; I'm looking to start a PbP game with a few others where players rotate into the GM role on a monthly basis. Hopefully it's not something we would end up having to worry about, but I'm trying to consider the effects of major problems PbP games commonly face.
If I recall correctly, the Powers That Be recently added co-GM capability, so two people can both have GM powers. You might consider setting it up so that you are always a GM, with the power to promote people in and out of the co-pilot's chair. Then if one of them goes AWOL during their stint, you can kick them out and put someone else into their slot.
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Every campaign has to have a GM named, though, so start as the GM and join the players as normal. When you're ready to promote, go ahead and it will switch you as a player in the campaign.
Are you confronted with such a situation now, or were you just musing?