Logging in will not be possible without a trust between the domains, as the web service will not be able to verify the user credentials. Local users should work, however. When logging in with a local user you'll have to add the machine name just like you would a domain. If for example your web service is installed on a computer called LAN-WIN7, then a user called User1 would log in with LAN-WIN7\User1. This user has to exist on the machine in question.