It is possible.

1. Draw a Rectangle over your Group or Component.
2. Select both the Group and the Rectangle
3. Rightclick and select. "Mask with Form"

It masks the Group or Component with your drawn rectangle

Select your group of shapes and instead of making them a group go to the Object menu > Path > Add (Cmd + option + U). This will create a 'Union' instead of a group and can be used as a Mask.

Credit goes to Reinoud Schuijers below, I'm just reiterating as it seems people are still having problems.

Years later it is not possible to mask a group?!? xD

I am primarily an XD user and only recently had the opportunity to learn and use Sketch for a project. One thing I found to be very useful, particularly in conjunction with the Material Design plugin was the ability to use any shape or component/symbol as a mask for objects above it in the layers list that also belonged to the same group.

This was a convenient way to use surfaces with their shadows as a base for elements while using symbols and other elements to modify that base surface. It allowed for a flexible, yet controlled way to make more complex components that could easily be changed at the root.

This arrangement also retained the ability to align the edges of the surface itself to the grid and didn't force the element to expand to encompass the shadow as XD currently does it.

the most effective method is changing to sketch

By "duplicate a symbol" I meant create new instances of the symbol without duplicating the symbol. New instances with different mask positions.

I'd like to create a symbol with a mask containing multiple assets. I then want to be able to duplicate a symbol but override the mask position to create new assets. E.g., I create an "Avatar" symbol with multiple avatar images in a mask. I then create new instances of the symbol but change the avatar image by shifting the mask in each instance. I don't seem to be able to do this now.

> Mask within a group is not working

You can Add the shapes in your Group to make a single "union" shape, and then use that shape as a mask.

>> To clarify; use the pathfinder to flatten multiple shapes in to one shape (with the 'Add' method, CMD+Option+U), and then that shape (consisting of multiple, separated paths) can be used as a mask.

Mask with symbols would be very great because it would work like paragraph styles in InDesign, you just have to change the shape of the symbol and every linked masked image would automatically change its shape

> mask with shape doesn’t work with symbols elements

Yes, for now you can Ungroup the Symbol first and then Mask with Shape. You can create a new Symbol for Masked Group, if desired.

> Mask within a group is not working

You can Add the shapes in your Group to make a single "union" shape, and then use that shape as a mask.

> Can we mask two shapes like a line and a rounded rectangle?

Yes. Place Mask over top of all of the other shapes and then Mask with Shape.

Hope this helps,
Randy

Can we mask two shapes like a line and a rounded rectangle?

Can we mask two shapes like a line and a rounded rectangle?

Yes I am having the same problem. Mask within a group is not working. I am trying to mask a line within a rectangle and wasn't able to do so