Do you wanted to make it where there is the "whole store" of Products "displayed" to everyone, but only available for purchase to members, with the exception of the "memberships" ... which would more or less "tease" guest into wanting to become members so that they can purchase things ...
This could be done by using the Group Pricing as a method to flag the customer as a "member" or "not a member" ...
Now, if you need Group Pricing for something else, this could still be designed based on another flag of some sort on the customers table ...
The function zen_get_buy_now_button manages what button or link is displayed ... this can be customized to manage whether or not a customer gets the Add to Cart/Buy now or a link to the membership ...
Is there one kind of Membership or multiple Membership types?
If multiple types of memberships are they based on "what products" can be purchased or a time period for purchasing "anything" on the site?