Creating an Umbrella Membership add-on is very simple. Once you've created your Membership product in Exchange, you just need to enable Umbrella Memberships.
Umbrella Memberships can be enabled by ticking the checkbox in the Umbrella Memberships product feature. That's it! Umbrella Memberships does integrate with other parts of Exchange to provide more advanced functionality.
The Quantity Discounts add-on by iThemes allows you to provide discounts the more seats a customer purchases. View the image below for an example of how we've setup Quantity Discounts in our demo environment.
If you'd like to limit the number of seats a customer can purchase, this can be done using the Purchase Quantity product feature. View the image below for an example of how we've setup Purchase Quantity in our demo environment. At this time, setting a max Purchase Quantity won't prevent a customer from purchasing multiple memberships to exceed the limit set in the Purchase Quantity settings. This is a feature we are actively working on, and hope to release it in the not-to-distant future.