Select your name at the top right corner and select "Groups".
Now select the Group that you would like to attach the Facebook page to.
Next, select Library at the left side of your screen.
Toggle the pages that you would like to use for that specific group.
Select save at the top of the screen.
Now select Library at the top of your screen.
Here you can view the pages that you have attached to that particular group.
Once at the campaign level, you may select any page that is attached to a specific group.