We have an odd issue where one of our GTM Plus users is using his account but he no longer shows up under Manage Users. We try to add the user, but we get the error message "Email <email address> is already in use. No users were created or invited."
I've never seen an existing user suddenly disappear from the Admin Center, but the error message is legitimate if they had an account login separately from yours. This can be resolved if you call into Customer Support to request a movement of the problem email -OR- the affected user may change their own email address on the active account in order for you to add them to your group account (this method will not transfer any schedules or history).