Adding our CRM users as contacts

I am trialling the Customer Service Portal and have just set it up.

I wish to assign Web Roles (including administrator) to our users in CRM so that they have certain rights on the Customer Portal. My users don't yet appear as contacts in CRM, so I can't assign them web roles.

Should they be added automatically, do I add them manually, or get them to sign up via the Portal registration itself? In my mind I may be confusing the entities of contact and user a little. Are our users a special kind of contact, or do I just simply add my company as an Account, and add my users as contacts?

Regards,

Brian