Add CRM User from a trusted domain

Hi

I have an account on a trusted domain to the domain that the CRM instance is in, there are a number of users from this domain that have access to CRM, but a number of accounts can't access across domains

Is there some AD group that accounts need to be a member of, or is there some other magic formula

Any help would be much appreciated

Regards

Andy

CRM 4, SQL Server and .Net developer using C#