It's quick and easy to add a sub trainer as a group admin!
Here's how you do it, starting from the "Account" tab on your Dashboard. Step-by-step instructions are below.
Step 1: Click the "Trainers" button on the left side menu of your "Account" tab.
Step 2: Next to the trainer of your choice, click the "Groups" icon under the "Actions" heading.
Step 3: Select the correct group from the drop-down menu, and click "Assign to Group" to save changes.
Now this trainer is set up as a group admin!