When you create a site group, any shareables that you add are shared between all sites within the group.
Open the Business Control Center.
Select Site Administration.
Select the Site Assets link.
Enter the name of a project and click Continue.
Click the + icon from the Actions menu and select Site Group
The New Site Group panel appears.
Enter the site group Name.
Select the Sites that should be in the site group. You can click and drag the sites from the left pane into the Sites in Group window. Site groups can initially be created without member sites where you add sites to the group later.
Identify the Shared Data that will be available to all sites within the site group.
Click Create to create the site group.