We have just set up a two-way trust with a remote domain. I can add individual users to groups and assign permissions, but I can't add groups like 'domain users'. It's very odd behaviour. I can put type the group, like UNDPPC\domain users in the people-picker and if I hit OK I get an error:"The user does not exist or is not unique." If I check the name before I hit OK it finds and underlines the name ok, but if I check again it's not found.
Individual users can be added fine. Needless to say, the user search doesn't find anything.
I've tried adding the domain to the people-picker using stsadm searchadforests but don't seem to have had much luck.