Glad you found Composite Lab and are enjoying it thus far.
Basically for the effect, you'll need to shoot your actor(s) on the greenscreen and then you can duplicate that footage or just take mulitple shots of it, and import them into Composite Lab. With the greenscreen videos, you can move them around your canvas and position siethem the way you want. Then you can simply key out the green footage, and import your cheering crowd video or still. That should do it. Compositing Tutorial #1Compositing Tutorial #2Clab Cloning Tutorial
and Cloning Tutorial 2
(not exactly what you want, as you're using a green screen as apposed to a non green screen background)
As far as a Green screen outside, that is one way to do it. After all, the sun is the best light source, and it's free. Just keep an eye on the position of the sun, especially if shooting throughout the day, because, well, it would be in the sun's nature to move a bit. Shooting outside is a good alternative to sometimes expensive and somewhat difficult lighting set ups in studios. For a studio lighting tutorial, please see this
one I made.
Hope that helps a bit!