I am running Windows Server 2003. There are a couple of user accounts that I would like to promote to Administrator accounts. I've tried several ways to do so, but I am still relatively new to setting up a server. If anyone has any ideas on how to go about promoting these users, I thank you in advance.

EDIT: I should probably mention that this a domain controller. I didn't realize that this changed the answer I was looking for. I apologize, like I said before I am new to the world of servers.

EDIT #2: I've added the users to the Administrator group like most of the answer recommended, but the users don't seem to have admin rights yet. I think this might be because they are also in the Domain Users group, which I can't seem to be able to remove them from.