Inviting other people to GTM
How to enable other people to edit your GTM account
Before you start
- Be aware that sharing access to GTM with full permissions will enable the invited person to alter your website's code. Only grant full access to your GTM account to people and companies that you fully trust.
- This tutorial does not cover the basics of how to create a GTM account and container. You can learn about that here.
1. Log in to Tag Manager
Go to tagmanager.google.com and log in to your account.
2. Go to Account's user management
On the list of accounts and their containers, find the container you want to work on and click the 3 dots near the gear icon.
From the drop-down menu that will appear, select User Management.
3. Invite an agent
You will land in the user management panel; click the button NEW. 
Now you're in Invitation settings. Select the email address of the user you want to invite and select Account Permissions. When you click "Edit" in the container permissions, you will allow the new agent to edit the tags in the account but will not enable them to publish the changes and altering your website's code. You will still be the one who has to approve all suggested changes in "Tag Manager".
After clicking the button, "Add", the user will receive the invitation to "Tag Manager".
Very important: Please do not assign Admin access to people that you do not fully trust or who are not bound to a proper agreement.