Free membership and invitation code

I have mutlisite, buddypress, pro site. I have membership active on one subsite and am installing it on another subsite.

I know membership and pro site run parallel and don't "connect"

The new membership subsite needs to be totally private to a certain few of my pro site members.

It also needs to be free, because these members already paid in the pro site section.

When I create the free level in the new private subsite, I experiment with test pro site members. The registration form tells me they are already members and to just log in instead of creating a new account. Then I click and they go right through to the welcome page. I love this feature on my other membership subsite, but, not on this one. Because I don't want just anyone going in to this site.

I disabled the global login.

I tried using the invite code as a work around, but, this is on the new member sign up side of the registration form and isn't accessible for an existing pro site member.

Ideas? Thanks in advance for your help!