Start by figuring out your baseline encounter for five players. That's going to be the easy part. Then figure out individual monsters you can add to the encounter that are the same level as the PCs. You might want to have a couple of options: maybe one elite (which is worth two PCs), one normal monster, and a package of four or five minions.
Add one of those options for each additional PC, or for two PCs in the case of the elite. You're just scaling linearly, here, which is not going to be perfect but should be good enough. If you get ten people, just double the initial encounter. More than ten: double it, and add option packages again.
I wouldn't worry about scaling for the roles. With 15 people, chances are the balance will be decent.
You'll also need to scale the map, if possible. 4e fights take a lot of room: if you can increase your map size by 50% for 10 people, and double it for 15 people, your players will be happier.
Since a really big fight runs the risk of bogging down, I'd be willing to end those fights a bit earlier. It might be good to introduce morale rules so that people feel like they've won rather than having you arbitrarily call the fight.