You can easily add users or groups as administrators or owners using the Quick actions pane.
How-To
Select Explore from the sidebar.
Select your site(s).
On the Quick actions pane under Owners or Administrators, click
Select a valid user to add as Owner/Administrator.
If more than one site is selected: You will be brought to an options screen.
Type in the name of your user or group, and click Apply.
If one site is selected: You can add the Owner/Administrator directly from the Quick actions Pane by typing the name of the user or group and clicking Save.
Note: It is important that your sites have valid users assigned as Administrators or Owners. If you aren't sure how to check this, refer to our Invalid Owners/Administrators report.