Membership 2 - Invitation Codes

Using Membership 2 on a very simple site to protect bbpress forums access. Works fine. Only one membership level in place (a free membership level named "residents").

I realize that the "invitation Codes" functionality is in beta, but I can't seem to get it to do anything at all. I'm assuming that activating an invitation code would cause the registration form to display a field to input the proper invite code. (It does not).

Also - (perhaps related) when an invitation code is created, there is an option for which membership the code applies to .. as we only have one membership level, the options are "all" and the "residents" .. if the "residents" membership is checked off during invitation code creation, the system reverts to "all" when the code is saved.

SO - my question.. what am I missing? Is there a setting to activate the invite codes? If so, I can't find it.