Adobe 12040118 - After Effects Standard Tutorial page 48

Dragging panel (A) onto grouping zone (B) to group it with existing panels (C)
Dock or group panels
1. If the panel you want to dock or group is not visible, choose it from the Window menu.
2. Do one of the following:
To move an individual panel, drag the gripper area in the upper-left corner of a panel's tab onto the desired drop zone.
Drag panel gripper to move one panel
To move an entire group, drag the group gripper in the upper-right corner onto the desired drop zone.
Drag group gripper to move entire group
The application docks or groups the panel, according to the type of drop zone.
Undock a panel in a floating window