Unable to invite a user

Common reasons why you can't invite a user

You're account is full: You'll need to upgrade you account subscription to add more users in Settings>Billing
Email already exists: To move users between teams go to user settings, find out how here
Email already exists but user isn't showing in Team Today: If a user signs up separately and domain registration isn't turned on then they will create a new company in Team Today not linked to your own. You can turn this on in Company Settings.
User hasn't accepted an invite: When you add a user an invite is sent, the receiver must accept the invite, find out more here