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