How to add a manager to approve accounts within the site?

You can add a manager for approving accounts on the site.
For this, you can follow these steps:
 
1. First, go to the Site > Settings > Standard Section - Check the box: Allow Self Sign Up and select the approval group, e.g. Approver.

 
2. Go to the User Management section > Site Users - Edit the user (Approver) who should receive the approval notification and make sure that the user belongs to the Approver group in the tab > Site Group Membership.
 
3. Go to the Email Notifications area > Search for the notification: Approve account request > scroll down and click on the Customize this message button > In the section: Who receives this notification, select and move the group from the list to receive this email, e.g. Approver.
 
Please note: All users in this group will receive this notification.