cannot add domain user to pc

Hi,
We have a problem on site where if we try and add domain users to local professional desktops it gives an error message saying that the domain is not available.What we do is we go into the control panel then goto users and passwords then goto add and then browse to the list of domain users and although it lists all of the users when we try and select one it gives the error message about the domain not being available. We are using ststic ip addresses and can browse the network normally. Any help appreciated. Thanks.