Add active directory user to linux group upon sign-in

I have Active Directory sign in working on an Ubuntu 12.04 box. When an active directory user signs in, I would like to add them to the group sambasharers. Where and how would I add this declaration?

I guess I should add that I am using Samba and winbind for authentication.

Simple and probably not scalable solution:

Assuming you've already set up your Linux box to authenticate to Active Directory, then you just add each user who should be in the group to the sambasharers group on the UNIX Attributes tab in Active Directory Users and Groups. If you don't see the UNIX Attributes tab, see this Microsoft KB article.