A Private Portal is a portal that is only available for viewing through invitation. Here is an example of what one looks like. All you will see is the Log in tab:
There is no tab displayed for Open Public Opportunities nor is there any tab for Past Public Opportunities.
From this portal, a Vendor may either login to their profile (to see potentially any projects that they've been invited to) or create a brand new registration.
When the invited Vendor logs into their account they will see the projects which they've been invited to under the My Opportunities tab:
If a Vendor hasn't been invited to any bids it will look like the screenshot below: