Yes, to setup a subgroup first visit the My Learners page and find the user you would like to place into a group and click the Edit button.
If you want them to be a member of a group, simple type the name of the group you wish them to be in into the "Member of (groups)" box and press the [Enter] or [Return] key on your keyboard. If the name you type does not exist then it will be created for you automatically when you save.
Enter your password and "Save Changes".
The group will be created automatically for you and the learner will be assigned to that group.
You will then be able to filter your learners list down to just members of that group:
When you want to add future learners into that group, just edit them and start typing and the group will be suggested. You can either choose the existing groups or continue typing the name of a new one.
If you wish to grant another user ownership of a group, follow the same procedure, but enter the group name in the "Owner of (groups)" box.
Granting ownership of a group allows the new owner to see and edit all the members of the group on their "My Learners" page