Aid another checks - everybody rolls Stealth to support the main Rogue. Those who succeed add nothing to the check, but gain the ability to use one main Stealth roll instead of their own Stealth. Those who fail actively hamper the Rogue, providing a -2 for each failed Aid.
I was thinking something similar, but wanted to reduce rolling overall. I figured to take the worst penalty and apply it, then add +2 for everyone else who was trained in stealth. Simple and hopefully quick.
Then I got to looking at the survival skill and though, oh, yeah...assess a penalty of -1 for each additional person because it makes sense.
It wouldn't, necessarily, be easy for a stealth focused character to get a bunch of people past a guard station (like you've probably seen in any number of action and fantasy movies/books), but it would be doable unlike now.