Do:
/pex user <user> group set <group>
This adds you to that group. -- This will give YOU the rank of owner.
/pex user <user> group add/remove <group> - remove yourself from the member group, or whatever, and make sure you're only in the OWNER group.
You only set someone to a group, if they're in the highest. If you're wanting like.. admins or mods, you have to add them to the admin/mod group, and remove them from the default group. Unless you've typed /pex promote <user>
Do that from console, if it won't work in game.
