Can you explain what you mean by automatically log someone in to a domain? Are you talking about loggingin in to one website form another website here or a windows Active directory type domain.

If you are trying to log into another website from this one then you would need to know the receiving websites username/password variable names and post the user to the site. however some sites prevent this for security reasons. You can do this both on the client and server side.

If you are using the later then you can use impersonation either through scripting or IIS or both.