Limit access to SharePoint only?

[riot]

I have a SharePoint Foundation 2010 server set up on SSL, it uses the same URL internally as it does externally. I need to give access to a few people outside our organization. I tried setting them up in Active Directory and giving them logon rights to only the SharePoint server (I don't want them logging into the computers here) but they were not able to log in. If I go back to giving them access to all computers, then they can log into SharePoint. I tried giving them access to the SharePoint server as well as the domain controller, and they couldn't log in again. What do I have to do to only give access to the SharePoint site? Thanks!