The account that created the campaign has the ability to elevate any player to GM status. Just mouse over the player's icon in the Campaign Details page for the campaign and you should see a pop up that gives the options to "Promote to GM" or "Kick Player." Keep in mind that there are certain functions that only the account that created the campaign has access to, such as editing the campaign details or using supporter features such as the transmogrifier. Also, the supporter level of the campaign as a whole is determined by the account that created it.
