ftp site on a domain

I have an ftp server in my dmz. I was wondering if it is best to leave that server on a workgroup or join it to my domain? If I put it on the domain, what ports should I open on my firewall to allow that server join the domain and to authenticate the users that use the ftp site?
Any advise would be helpful. my ftp server runs on a windows server 2003 platform. Right now, the dmz is configure for my domain to talk to the ftp server but the ftp server is unable to talk with the domain.