In order to create a Fax Portal, you first must enable the Tenant Portal. You can read about that here:
Once you have created your Tenant Portal, follow these steps to make a Tenant User that is able to only access the Fax features on their account.
Create User and Assign Permissions
Navigate to Tenants -> Users:
Once here, click Add Tenant User:
Now, you have to complete the user information and assign permissions only for the Fax features:
Lastly, simply add the fax numbers that this Tenant User should have access to:
Now that you have created this user, they are able to sign into your Tenant Portal and send/receive faxes only on the number(s) that you specify.