A site groups enable publishers to group multiple sites together, so that anyone buying campaigns can target multiple sites with the click of a button when creating a line item or creative.
How to create a site group.
To create a site group, go to Inventory > Site Groups and click "New" in the upper right corner. Give the site group a name, a description (optional), and any labels (optional) to make search and reporting easier. Then click save.
Creating a site group.
Then, go to any site that you would like to put into the site group. Under Site group, click the dropdown menu and choose your site group. Then click to save the changes to the site.
Each site you create can be placed under one of your site groups.
After this, when you or any buyer clicks to target a line item or creative, they will find your site group as a criteria. Once a site group is chosen, the line item or creative will target that group of sites.