The full list of controls can be accessed by using the Settings gear icon from the top-right corner of the Assemble web page.

General Controls
I : Hide/Show Details
C : Add/Remove Columns
B : Edit Group By
< > : Expand/Collapse Grid Level
ESC : Unselect All : Unselect all checked rows in the grid.
CONTROL + mouse click : Add to or Remove from Selection : Control click a row to append it to current selections, or remove it from current selections.
Viewer Controls
U : Toggle Measure Format : Toggles the measurement format between Decimal Feet, Feet-Inches, and Decimal Meters
Q / SHIFT + Q : Quick-Hide Current Selection : Temporarily hide the currently selected objects in the 3D Viewer. Hold shift to unhide the objects. Does not affect Inventory or Sheets.
V / SHIFT + V : Toggle Orbit Selection Mode : Toggles orbit about selected objects. Hold shift key to toggle hiding unselected objects.
X : Toggle X-Ray Mode : Toggles the view of the model between X-ray (transparent) mode and an opaque state.
G : Toggle Ghost Mode : Ghost mode allows filtered out objects to be visible in the model, but not visible in the Grid and not selectable. After applying filters you can choose to see the remaining visible objects either free-standing (Ghost mode off) or prominently displayed with the rest of the model visible, but "grayed out" (Ghost mode on). Ghosted objects can only be selected by deleting the filter that suppresses them.
O : Toggle Orthographic Mode : Orthographic Mode offers a plan view or elevation view of your model. While in Orthographic Mode, the Assemble Model Viewer displays "Orthographic" in the top-left corner of the Viewer window.
CTRL + S : Save a screenshot of the Viewer in PNG format.
+ / - or scroll wheel : Zoom In/Out
Z / SHIFT + Z : Zoom Extents : Zoom out to reveal the full extents of your model. Hold shift to zoom to your selection.
H : Go To Home Viewpoint : Return to the original starting viewpoint of your model.
0-7 : Change Viewpoint : Switches camera angles in the viewer.
Mouse click / SHIFT + mouse click : Select Instance(s) : Selects one or more instances in the viewer.
Mouse right click drag / CONTROL + mouse right click drag : Marquee Select Instances : Selects everything that is completely contained in the rectangle. Hold down Control to append to the current selections.
Dragging left to right will only select instances which are fully contained in the selection rectangle. Dragging right to left will select any instance that is partially within the selection rectangle.
ALT + mouse right click drag : Marquee Unselect Instances : Unselects everything that is completely contained in the rectangle.
Dragging left to right will only unselect instances which are fully contained in the selection rectangle. Dragging right to left will unselect any instance that is partially within the selection rectangle.
SHIFT + mouse left click drag / Mouse wheel click + drag : Pan Model.
CTRL + LEFT / CTRL + RIGHT / CTRL + UP / CTRL + DOWN / Mouse left click drag : Rotate/Orbit Model : Rotates the model using the mouse or keyboard.
W / SHIFT + W : Walk Forward : Walks the virtual camera forward in the model. Hold shift to run.
S / SHIFT + S: Walk Backward : Walks the virtual camera backward in the model. Hold shift to run.
A : Turn Left : Turns the virtual camera left.
D : Turn Right : Turns the virtual camera right.
R : Look Up : Rotates the virtual camera to look up.
F : Look Down : Rotates the virtual camera to look down.
UP ARROW : Move Higher : Increases the virtual camera altitude.
DOWN ARROW : Move Lower : Decreases the virtual camera altitude.
LEFT ARROW : Slide Left : Moves the virtual camera left without turning.
RIGHT ARROW : Slide Right : Moves the virtual camera right without turning.
SHIFT + PLUS KEY / SHIFT + Scroll wheel: Increase Field of View : Widen the virtual camera’s field of view (telephoto zoom out)
SHIFT + MINUS KEY / SHIFT + Scroll wheel: Decrease Field of View : Narrow the virtual camera’s field of view (telephoto zoom out)
Sheets Controls
+ / - or scroll wheel : Zoom In/Out
Z / SHIFT + Z : Zoom Extents : Zoom out to reveal the full extents of your model. Hold shift to zoom to your selection.
H : Fit Width : Fit the sheet to the width of the window.
Mouse click / SHIFT + mouse click : Select Instance(s) : Selects one or more instances in the viewer.
Mouse right click drag / CONTROL + mouse right click drag : Marquee Select Instances : Selects everything that is completely contained in the rectangle. Hold down Control to append to the current selections.
Dragging left to right will only select instances which are fully contained in the selection rectangle. Dragging right to left will select any instance that is partially within the selection rectangle.
ALT + mouse right click drag : Marquee Unselect Instances : Unselects everything that is completely contained in the rectangle.
Dragging left to right will only unselect instances which are fully contained in the selection rectangle. Dragging right to left will unselect any instance that is partially within the selection rectangle.