Cannot create a user

I'm trying to create a new user and everytime I click 'OK' it sends an invite rather than adding that user to the system. I've tried multiple times to delete the invite and add them again but each time it just sends an invite instead of adding them.

I've tried multiple browsers and reset my cookies/cache. Is it something to do with the username I'm trying to use? E.g. personal@mydomain.co.uk? If I try other names such as test@mydomain.co.uk it seems to work fine.

Thanks