There's two different ways to invite an Admin into the back end of the system. Owners have an ability to invite admin into the the system through the Admin Management system. All other Admin can invite through the User Management Module.
Access the Admin Management module go to the "Settings" tab and click "Admin Management"
Click "Invite New Admin"
All other admin:
All other Admin (Full Time, Part Time, and Guest Admin) will need to invite Admin from User Management. Click the "Actions" box then "Invite Admin"
After those steps the process for both Owner(s) and other Admin(s) is the same. A module will open to enter their email address then enter specific details (First and Last Name, Billing Type) then click "Send Invite"
NOTE: Any non-Owner Admin(s) can not invite any admin above their level of access. This means Guest Admin can only invite other Guest Admin and Part Time Admin can only invite other Part Time Admin.