If you are part of a network, you can create collections in the Start Organizing menu.
Collections are ad-hoc lists of child groups in your network, allowing you to easily select them as a group rather than having to choose them one by one.
On the collections page you will see a list of your collections for your group. You can edit or delete collections if you want, or click "Add New" to add a new collection.
Give your collection a name and then choose which groups you want to be part of the collection from the interface. Save your collection when you're done.
You can use collections wherever you have the choice of child groups. For example, on the network groups filter when targeting emails or reports, you can choose to add a collection to the filter. This will bring up a dropdown menu with a list of all of your collections. Choose one to add all groups in it to the network groups filter, on top of whatever else you already have selected.