Which rights are you referring to? The following?

Go to the Family Groups page. One easy way to do this is the drop down menu under your account icon:

Click on the group you want to modify then click on the Group Members tab. There you will see a list of all the members and off to the right a More icon for each member:

There is not much to edit.

An administrator can only:

• Be changed to a member. (This can only be done if there is more than one administrator.)
• Leave the group. (If there is only one administrator and he or she leaves the group, the group is either deleted or maybe just archived. In any event no users have access to it anymore. I have no idea what actually happens to any profiles that were created in it.)

A member can only:

• Be changed to an administrator.
• Be removed from the group.