Ask for a view to the Dashboard log for clues to what is happening. It could be active directory related. There may be a long list of users, and the user already has access. 

Hi,

I have found the issue but not how to solve it, when I fore example add user domain\marcus I get an error that multiple users/groups ave been found, we have multiple users that start with domain\marcus. How to I specify the specific user?

Has this always been working, or is this a new error? An iisReset could be in order if it was working prior. OR, you would want to check if the CP Configuration Manager (IIS Settings) is setup to use the correct Domain. Also look in the Security tab of CP Config Manager if 'Use Active Directory' checkbox is checked.