I've setup the server. Opendirectory LDAP is running. Created a group called ArchitectureOffice and i've got 4 accounts as members of that group. Created a folder on the Server HD under Shared Items, called OFShared. Configured a SMB share point with both POSIX and ACLs to allow the ArchitectureOffice group access to that folder.
A few windows xp machines are able to connect successfully to the server, each using their own password and username.
BUT
When they try to write anything to the SMB it gives them a 'Disk is full... error' sometimes an 'Access denied'. I don't understand what's going on, shouldn't everything i copy to that folder be accessible to everyone in the ArchitectureOffice group?
check your logs on server. Is it possible that SACL is blocking your users? Start up Server Admin, click server name and select Access icon. Now check if your users are allowed to use SMB at all.
Since your clients are running XP, you'll need to add these directives per share in /etc/smb.conf:
Beware that I believe this breaks smb/cifs ACL support on the OS X client if that matters to you, because it effectively tells the client to ignore ACLs.
More info here: http://www.stanford.edu/group/macosxsig/blog/2010/01/acls_not_being_properly_honore.html