How do I mask a group in After Effects?

To mask a group in Adobe After Effects, you will apply a mask to a layer. This will then reveal or hide parts of the layer. Here's how to do it:

Steps for Masking a Layer in After Effects

Select the Layer: In the Composition panel, click the layer you want to mask or make sure the target layer is displayed in the Layer panel.
Create a New Mask:
- Go to the top menu and select Layer > Mask > New Mask.
- Alternatively, you can use the keyboard shortcut Ctrl + Shift + N (or Cmd + Shift + N on a Mac) after selecting a layer.
- A new mask will appear on your selected layer with handles surrounding the outer edges of the frame in the Composition or Layer panel.
Adjust the Mask:
- Change the mask shape:
  - Select Layer > Mask > Mask Shape. The default shape is a rectangle.
  - Click on the mask path, which has small squares at each corner and side, to adjust its shape by moving the squares.
- Choose your mask tool: Use the pen tool or shape tools to draw your mask directly on the layer within the composition or layer panel.
Mask Operation:
- After creating a mask, you can change its operation to Add, Subtract, Intersect, or Difference from the layer's properties within the mask panel.
- Add: Shows the content within the mask.
- Subtract: Hides the content within the mask.
- Intersect: Shows only the area where the masks overlap with the layer.
- Difference: Hides the area where the masks overlap with the layer.
Feathering and Expansion: You can feather your mask and also expand or contract the mask edge to refine the mask's appearance from the mask properties on the layer itself.

Practical Insights

Multiple Masks: You can apply multiple masks on a single layer. Each mask can have its shape, operation, and properties. This allows for complex reveal and hide effects.
Mask Path Animation: Masks are not static. You can animate a mask by adjusting its path, feather, or expansion over time, creating compelling effects.
Parenting masks: You can parent masks to null objects or other layers to maintain a dynamic mask that follows the movement of the parent layer, which can be useful for complex mask animation.

By following these steps, you can effectively mask groups (layers) in After Effects to achieve your desired visual effects. This method is also useful for selectively revealing or hiding parts of layers.