User activation issue

I have now two users I have added to one of my projects, and after signing up and attempting to log in they get the following message:

You have been deactivated in this account

To regain access to your projects, your admin will need to reactivate you.

Please contact your admin (admin@myaddress.com - not our admin real email here) to activate you and restore your access.

How can I activate a user? We've never had this problem before.