Currently when you invite someone to group while in Dalaran, you are not on the same phase and do not become phased at all, even after waiting for 10 minutes. You have to run to Violet Hold to do a trade with your party member.
Same issue in other zones like Dragonblight and Icecrown. You cannot see other people in your party in the zones. This sucks for people trying to level, or trying to trade each other tailoring mats at the sherines.