Adding new group with new users in security properties, unable log

I'm using Windows NT Server with IIS.
Everything was preconfigured, I have a site with restricted access, I
created a new local group with new users. I added this group in the security
properties of the folder but when I try to access in this page I can't.
I try to logon on my machine as one of this new user and I can access to
this share folder.
At the end I try to add one of this user in an old group and everything
works, I can access the page from iexplorer. I need to know why an old group
works an the new one I made doesn't work.
I chek all the properties in IIS and also the folder properties but I can't
find the problem.
Tanks. Alessandro.