Training stealth up from zero is generally pretty difficult because you need to pass a stealth check to actually gain skill. The best way is to just find someone who already has stealth skill to recruit and have them teach others. If you really want to train up from scratch, the best way to pass stealth checks is to wear clothing that's marked "VERY SNEAKY" and have people wander around solo - in a group every character rolls checks individually and if any of them fail, they all fail.