This process of adding Managers to a Google Plus page has changed recently and we wanted to get quick updates out of how to do this.
Ensure you are logged into your Google Account.
Go to your Google+ page
In the upper-right, click the arrow button and you will see a link called “Managers” that appears in the drop down. Click that link.
You should see a box titled “Add Managers by email”. Just enter the email address of the person you want to invite and click “Invite”. If you do not see this box, then you do not have permission to add Managers.