The Real ID Party System was announced less than year and since than we heard only few news. This system was in alpha and beta testing and now its live.
To use this system you will need to have Real ID account, you must logged in and you are ready. The system allows you to invite 4 players + you = 5 of same faction for normal or heroic dungeons. For this system it doesnt matter are you in this realm or that realm you can invite anyone who has Real ID account and logged in.