First of all, it is good to know that you can add as many administrators as you need.
To invite a new Admin, you can easily do that by the 'Admin' view from the left menu. Then click on 'Invite a new admin' on the top left corner of the page.
After clicking, you will be asked to enter the email address of the person you want to grant access as Admin. The system will send an invitation email to that address to set up the new Admin profile.
It it not possible to add more administrators simultaneously.