Apple Motion 4 - Manual (Page 384)

The boundaries of that one object are highlighted in the group track.
2 Move, trim, or slip the object independently from the other objects in the group.
Trimming Objects in the Group Track
Trimming the edges of the group bar automatically trims the edges of the objects within
the group. If there is only one object, trimming the group bar simply trims that object. If
there is more than one object lined up with the edge of the group, trimming the group
automatically trims all of those objects.
384 Chapter 8 Using the Timeline