The following guide will walk you through adding someone to your Google Tag Manager account so they can make edits or troubleshoot the functionality.
Login to your Google Tag Manager Account https://tagmanager.google.com/
Click the three vertical dots next your your container name and click User Management
Click the blue circle with a plus sign and then click Add Users
Enter the email address (the email address needs to be linked to a Google Account) you would like to provide access. Next, choose all the permissions checkboxes shown below. Then click Invite button in the top right of the screen.