Ubuntu Server is a really great distro, and its even better when you have a centralized, single sign on user and password for all the people using your server.
This whitepaper will teach you how to configure your ubuntu server to authenticate users using an Active Directory, so you can have a samba folder access restricted to only a certain group or users (for example), or have everyone log in onto your jabber server for company chat using the same login credentials.
This will also work if you just wanna join an active directory as a user, both scenarios are covered on the whitepaper.
The document is really complete and very well explained, you will have everything you need to understand and setup your server or host with active directory integration.
Enjoy!