You can add a booking to a roster group while you're creating it, or add an existing booking later. If a booking has more than one trip, add each trip separately.
On a new booking
Create a new booking as usual.
In the Itinerary builder, click Add to roster group and select the group.
Finish the booking as usual: save it, confirm it, or send the invoice.
On an existing booking
From the Roster groups tab
Click Trips in the left sidebar, then click the Roster groups tab.
On the group's card, click Edit Group.
Check the itineraries you want to add, then click Next.
Click Save group.
From the Trip calendar
Click Trip calendar in the left sidebar, click the trip, then click Manage booking in the pop-up.
In the Itinerary builder, click Add to roster group and select the group.