I'm attempting to add a ton of users via a text file so I don't have to type things in manually on every machine.
Are we to assume from that that they're local users not domain users?

Unless you have some special reason you are probably better off adding them to a domain (NT4 or Active Directory). The "net user" command mentioned by Noia will work to add local users or domain users depending on the options.

You can add users en-masse using a simple script that uses "net user" - for instance a batch file or perl script.