Select your name at the top right corner and select "Groups".
Now select the Group that you would like to attach the catalog to.
This feature is particularly helpful for Agencies that create Groups as containers for each of their client ad accounts.
Next, select Library at the left side of your screen.
Toggle the catalogs 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 catalogs that you have attached to that particular group.
You can see which catalogs are “connected” and those that are not.
Once at the campaign level, you may select any catalog that is attached to a specific group.