We could start by sending in our mugshots so we have a list of looks suited for certain sketches, and from then on keep the sketches as basic as poss.
It's best to think up the sketches like having two people on a park bench - minimal background, costumes and props but with memorable dialogue.
"The Sweet & Simple Show". It won't make Channel 4 but be good for a viral?
The hardest part is getting people up off their 'writers chairs' to actually do something that isn't writing. Like Dave C says, for him it's totally out of the question. Many feel the same way.
You need a focal point of some kind. The way we did it was that one of our group put up a thread and suggested a meet and a deadline to say 'yay' to a meet. The ones that said 'yay' were then invited to meet. The ones that turned up were in the group. QED. The catalyst that made it move to the next step each time was the deadlines. Literally, if you didn't turn up to the next step, you were out, unless you had a good reason (for us there were only two good reasons.. birth or death).
On here, you could do it by starting a thread that says: PM me for my email addy and then send me a recent pic, 3 sketches ('do-able' with locations) for the pot and a profile on what you can do, 'equipment' you've got (easy Leevil..) and location/mobility e.g. actually prepared to leave my house and go beyond the local shops to turn up to a shoot. The ones who are able to respond to the challenge are in the group. Some members may only be able to submit scripts, like Dave C. Fair enough, depending on the rules of your group.
Anyway... like we always say, what the f**k do I know, what worked for us (in terms of getting a filming group together) may not work for you!
Good luck! 
Frankie xxx