Sharing access in Google Tag Manager is a straightforward process; it doesn't require sharing your username and/or password. Take care that only the necessary level of access should be granted.
Container will not be visible. No level of access.
Can view container, tags, triggers, and variables, but can't make changes.
Can create workspaces and make edits, but cannot create versions or publish.
Can create versions, workspaces, and make edits but not publish.
All creation permissions including publish.
Compared to Google Analytics, there is no email notification for newly created permissions.
That is it. Permissions can be edited/revoked from the User Management screen.