ok, explain more then, what OS are you running? Are you workstations setup in a domain? Are you running Activie Directory?
Generally how you would accomplish this is to, assuming you are running a domain, create a global group. Add all of your users to this group, then create a Group policy object in AD which grants that global group the right to logon to the workstations. That is the simple explaination. I guess to get started you should check out microsoft.com and start researching domains, AD, user right assignments....HTH