[ubuntu] Help with virtualbox shared folder permissions for ldap users/groups

Really need some help to get the permissions sorted for ldap users on the guest OS to access the shared folders.

Here's the scenario:
Host: Ubuntu Server 10.04 LTS 64 bit with Virtual box 4.2.6
Guest: Ubuntu Server 10.04 LTS 32bit with Zentyal 2.2 & Guest Additions installed

I have a directory on the host shared with the guest. I want to use the shared folder as a smb share for the guest users. Permissions on the host directory are 777. Permissions on the guest VB shared folder are 770, owned by user root and group vboxsf - I can't change this.

The users on the guest are authenticated via ldap and all belong to the ldap group __USERS__

My question is, how do I add the ldap group __USERS__ to the local group vboxsf to allow them to access the shared directory? Alternatively, is there an automagical way to add new ldap users to the vboxsf group?

Thanks