SQL Server 2005 Adding a User on a different domain

Hi,

Can anyone point me in the right direction with a problem I’m having.

We have two domains, with the DB server on domain A. I can add any user in domain A to the db without any problems. If I search for a user in domain B it finds and qualifies their username but when I click ok to complete the task I get error :-

Windows NT user or group ‘DomainB\Fred.Bloggs’ not found. Check the name again. (Microsoft SQL Server, Error: 15401).

Is this something to do with trusts between the two domains?

Thanks