Hi Rachel:
I think you may be confusing two different actions, registration and login.
It seems to me what you want to do is register the user with the properties that you need to check and then use the registered information that you get during registration to verify that the visitor is a member.
If you follow the process and most of the code here you can do just what you are looking to do.
For your purpose you probably don’t need to use the wix-users or wix-crm APIs as all these really do is provide a more secure registration and login/logout process that forces you to use the Dashboard CRM. While this may be more secure, it is more difficult to use and highly restricts what you can do without interacting with the dashboard interface.