How do I open up my WAN

I am in charge of a Three site NT4 WAN on TCP/IP. I need to open it up so that clients can share data more easly. I keep coming up with the same problem, the clients get asked to supply a password to use the resource, there is no password that will fit, and I have not set one up. It is not a permissions problem as for a test I set up a client Win95 with administrative permisions and they still were un able to use data on the other servers. I have two computers running Win98 that have no problem is this a OS problem? The four sites are set up as seperate domains, I do not have the time to make them into a single domain is there another way?
Thanks
Alex