We plan to add an official feature in the future.
In the meantime, you can break the signup process into two steps. We're going to need to create a free membership for each of our paid memberships, and use the free membership as the initial signup links. When a new member "signs up" for a free plan, we'll know they started the signup process but didn't finish checking out.
Step 1 → Create a paid membership. Let's call it "Pro" for this example. Add the signup links to "secret" pages on your website. We'll link to these pages in the next step.
Step 2 → Create a free membership. Let's call this one "Pro Signup." The "Page after signup" URL should point to the page you created in the previous step. Add the signup links for this new membership to public pages on your website (e.g. sample pricing page).
Step 3 → Crap... how do they access/do things with this data?