Do computers have to be in a domain

I have w2k server and clients and

Strangly the computers are seeing the server and each other without adding them to a “domain” in AD In the w2k clients they are setup with the defualt workgroup name for now I thought they would HAVE to be addedto the domain to “see” server and it’s resources so they aren’t logging into server but are able to see it map drives and share resources will this become a problem later