It basically allows you to do the hiding/skulking trapping from stealth with deceit and in situations that would normally not allow for it. If you do it for a group, that group has to stay together as close as possible while under the veil, if someone gets too far away, they become visible again. The person you are veiling does not have to resist the spell, unless they do not want to be veiled, of course. Anyone trying to perceive the veiled person rolls against the deceit roll you made when veiling the target. You are right about the casting stress, but veils usually last for one scene.
You example is pretty much on the target. You roll deceit and get +3, which will be the target roll for anyone that is trying to perceive the target. The guard rolls +0 and can not perceive your veiled friend, but maybe there's a dog next to him and he rolls +4. The dog can see (or smell) you, while the guard is looking around, puzzled as to what has the dog on edge.