Most of the items you're advertising only you (as the one doing the inviting) receive as a reward. The people who use your codes (your "friends") will not get rare dyes, mounts, etc. All they can receive is an item that grants a 20% XP boost when used on a class below level 25, 99 free teleport tickets, a fist-bump emote, and few tokens (feathers) which can be traded in for specific equipment that can also be obtained through simply playing the game.