server 2003 folder shares

I think I have lost the plot here, but..I have a number of folders shared on a file server, I also have a number of global security groups with members in. My understanding is that if i grant access for a group in the folders ACL then any member of that group shold be able to access the folder share, is this correct?

I have checked the effective permissions and the individuals with the sec group show as having the correct level of access. Yet when ever they try and access the share they get a denied message, any ideas?

Dean
PS there are no denied priviledges on any of the shares