Go to the 'Settings' tab on the left of your account dashboard.
Click on 'Users' and you'll be able to see the 'User Management' heading.
Type in the email address of the user you would like to extend admin rights to. Once the 'Invite' button is clicked, they will receive an email inviting them to claim their admin rights. The new user must follow the steps in the email to gain access to their admin rights. You can add as many admin users per charity as you want!