Status:
Answered
If the Coupon Module is enabled, it would be nice if it put an option in when you are creating a Membership plan to require a Coupon Code. This way, only the people you invite or give the Coupon Code gain access to the content related to that membership plan. If a zero dollar amount is also enter for the plan then all that should display on the checkout page is a field asking for the Coupon Code (Kind of works like an access code). No Payment option or Continue button should display. Once they supply the Coupon Code, then the Continue button can appear and they can finish the process.
This is already supported by the Affiliates Suite module, which essentially requires that a user be referred by an affiliate in order to register. You can pair that with the Affiliate Coupons module to let users enter a coupon given out by an Affiliate that will then mark the user as being referred by that affiliate, and let them then proceed.
-Leighton