On the top portion of the Contact/Customer page is a check box: Allow this contact to log in to the 'Your Company Name' company.  Check this box and Apply Changes for the page.  (An email is automatically sent inviting your new user to join your company.)

Once you create a user, you can select the edit link which will open the user access page.  (Edit link is located in the same place you found the check box to invite this contact to join your company.)  You can select up to 5 different access levels.  Level 1 access is assigned by default.

Hint:  On the Company User Access page you can send an email to re-invite this user to join your company.