you would have to edit /etc/samba/smb.conf
or if that is confusing, download and install webmin
http://webmin.com/download.html
uncompress it and as prompt, cd into the webmin folder and sudo setup.sh, follow the instructions
then browse to http (or http(s)://localhost:10000, login as the user/password you defined, browse to servers.. samba windows file sharing
click on 'windows networking'
set 'workgroup' to the name of your domain by filling in the blank and setting the radio button next to the blank
set 'security' to domain, save settings
then click on 'bind to domain'
and use the domain credentials necessary to bind to said domain