Having a bit of s samba config issue. Running RH 6.4, samba 3.5.10.
Samba host has joined the domain (net rpc)

The problem I'm having is that if I spec a user id (valid user) in the share, I don't get prompted for a password when trying to access it from a Windows host. If however in the share options I spec a group name (ie. valid user = @aiswf), I get prompted for an account and password. The user accounts and groups have been created on the samba server.

Why is a group share access behaving differently from a user share access. Am I missing a config/setup step ?
Below is my smb.conf

[global]
        workgroup = DEVENG
        server string = %h Samba File Server %v
        hosts allow = 127. 10. 172.
        smb ports = 139 445
# --------------------------- Logging Options -----------------------------
        log file = /var/log/samba/log.%m

        max log size = 20
        log level = 3
# ----------------------- Domain Members Options ------------------------
        security = domain
        encrypt passwords = yes
        passdb backend = tdbsam
        password server = *
        domain master = no
        wins server = 10.XXX.XX.XX,172.XX.XX.XXX
        dns proxy = yes
        deadtime = 10

# ----------------------- Browser Control Options ----------------------------
        local master = no
        preferred master = no
        os level = 33
# -------------------------- Printing Options -----------------------------
        load printers = no
        show add printer wizard = no
# --------------------------- Filesystem Options ---------------------------
        map archive = no
        map hidden = no
        map read only = no
        map system = no
        store dos attributes = yes
        hide dot files = no
#============================ Share Definitions ==============================
[neteng]
        comment = Network Engineering Share
        path = /fs_neteng
        browseable = yes
        writable = yes
        valid users = @neteng dignarn
[aiswf]
        comment = AIS Workflow Share
        path = /fs_aiswf
        browseable = yes
        writable = yes
        guest ok = yes
        admin users = arnoldr
        valid users = @aiswf