The source code is here.

I tried out 3 different approaches to get this working fast and accurate:

• Subfrustum selection is fast, but inaccurate.
• Projecting unit bounding boxes onto screen is slow.
• Project unit bounding spheres onto screen is fast and accurate.

Check out these links and Stack Overflow questions for more details:

• Marquee Selection with Threejs
• StackOverflow: Three.js: How to detect the intersection of a 2D square and 3D object
• StackOverflow: Three.js: How to pick all objects in area

A screenshot follows: