Cannot add user!

I have added a domain, but when I try to add a user for this domain I receive a red error message (which disappears so quickly I can't read it). I have since noticed that the domain in question (timwhitingphotography.co.uk) appears in the 'User Details' in the 'invitation list' tab and is pending!?! This is a business account that needs to be live for me to make a living

Please help!!!