You select you GM name from the top drop down, then Other Human Managers->Add Edit Managers. On the right select your manager and on the left you can check the Play in Commissioner mode box. This is where you can create more GMs also.
I would recommend creating a GM, giving them commissioner powers and not hiring them to run a team. The commissioner has some advantages a normal GM does not. There are also mistake (delete player) that are available in commissioner mode that can make for painful mistakes. I usually have my GM and an extra GM that is the commish.
I always name my commissioner account "Rick Hummel".
" Known throughout baseball by his nickname "The Commish", he is a former President of the Baseball Writers' Association of America.[3]"
https://en.wikipedia.org/wiki/Rick_Hummel