To add a new user to the business manager, you need to go to the "People" section, click the "Add" button and enter an email address (not necessarily a real one).
Then select the access rights that you want to grant to the new user, usually administrator rights.
After entering the email address, an invitation link to it will be sent. However, to get the link to the invitation, you need to click the "resend" button, and Facebook will give you this link.
Click on the link on the new social account and accept it to complete the process of adding a new user.
