A Resource Group is a collection of Resources, e.g. "Members Only" pages.
You can add or delete members of a Resource Group in two different ways:
Go to Security -> Resource Groups. From there you will see a tree of Resource Groups and a tree of Resources.
then drag the Resources from the right tree directly into Resource Groups in the left tree:
When you are editing a Resource directly, click on its "Access Permissions" tab, and check which Resource Groups it is a member of.