On your C9 Layout, Group Broadcast buttons are labelled with a unique icon (Figure 1):
Figure 1 Broadcast button
A Group Broadcast to multiple counterparties is initiated by simply selecting a Group Broadcast button. When selected, the broadcast button and all assigned counterparty connections are illuminated to give the user visual feedback on who they are actively broadcasting to (Figure 2):
Figure 2 Active Broadcast
You can dynamically add or remove counterparty connections to the Broadcast Group on a one-off basis. Simply right-click the button and check or uncheck the relevant box as required (Figure 3):
Figure 3 Add or remove counterparty connections