What are some simple video editor with Masking capability?
An oldie question, but let's add an answer...

The answer depends on if the faces are moving through the frame.

If the faces are relative still, like a camera on a tripod and the persons sitting in front of the camera, any video editor with more than 1 video channel can do the trick. Just add a layer with something on it, a prepared semi-transparent PNG as a mask perhaps.

But more complex software is needed if the faces are moving or if you want a blur, instead of a semi-transparent mask.

If the faces are moving you need to track them, so the program needs to track it.

As @Shultc commented Both Blender and HitFilm can do this, and I would add Davinci Resolve.