Membership2 How to use Invitation Codes

We love the invitation codes for Membership 2. We tried to use them in beta but had problems creating them.

Now we created 2 invitation codes that should allow specially invited people to access non-public membership levels. However, we can't figure out where the invited people would go to enter the code to register and select the hidden, non-public membership.

We have 2 publicly available memberships and during the activation of invitation codes, we saw that our Founder Membership was restricted and required an invitation code. However, I need to know how to get a user from the Register page via code to a hidden Membership level (Seminar and Gratitude levels).

Our Registration Page:

I read other threads that didn't really answer the question:
- Someone asked how to email it so the code is encoded in the link. (This would be awesome!)
- Another person was told to just have the people register and enter the code there, but there isn't an invitation code input on the register page.