Apple Motion 4 - Manual (Page 363)

A black border highlights the group.
2 Release the mouse button.
The object moves into the selected group and is placed above any existing objects within
that group.
You can also drag an object to a specific place in the new group by dragging it in between
existing tracks in the new group. Motion’s groups and layers are “spring-loaded,” which
means that when they are collapsed, dragging an object onto them causes them to
temporarily expand, just like folders in the Finder.
To drop an object inside a collapsed group
1 Drag the object onto the collapsed group in the Timeline layers list.
2 Position the pointer over the name of the group until the group springs open.
3 Drag the object to the desired location within the group, then release the mouse button.
363 Chapter 8 Using the Timeline