According to the group mapping section of the Samba HOWTO Collection you need to set up the default groups if you are setting up Samba for LDAP. Remember to set up your file first and then do something like this:

/usr/share/samba/scripts/ -g 512 domadm
/usr/share/samba/scripts/ -g 513 domuser
/usr/share/samba/scripts/ -g 514 domguest
/usr/share/samba/scripts/ -g 515 domcomps
/usr/share/samba/scripts/ -g 516 domconts
net groupmap add rid=512 ntgroup="Domain Admins" UNIXgroup=domadm
net groupmap add rid=513 ntgroup="Domain Users" UNIXgroup=domuser
net groupmap add rid=514 ntgroup="Domain Guests" UNIXgroup=domguest
net groupmap add rid=515 ntgroup="Domain Computers" UNIXgroup=domcomps
net groupmap add rid=516 ntgroup="Domain Controllers" UNIXgroup=domconts
/usr/share/samba/scripts/ -a -u 500 -g domadm -n -A 1 -N Domain -S Administrator Administrator
/usr/share/samba/scripts/ Administrator

Please note that this does not set up the complete groups that Windows DC’s provide, but gives enough to start with a basic system and you can build it from there.