To LOS: That's correct. You can use a Warlocks pet to help lower level characters get full XP even if the mob is not green to the Warlock. You MUST have at least 2 lowbies in a group. It does not work when you have just one lowbie.

Edit: instead of two lowbies, you can group a high character with a lowbie. Keep the high character outside of XP range from your kills, but close enough so that WoW still sees them as grouped. Your lowbie will get full XP for the kills.

Edit: BTW, I don't take credit for discovering this. It was discussed in some post on this site some time back, but that post is probably buried pretty deep now. But I'm sure taking advantage of it!!!!