Group Level RestrictionsFollow

Anyone have any knowledge on the restrictions, or lack thereof, of levels within a group? I grouped with my level 11 friend while level 17, whiched seemed on the cusp of the old EQ rule of lowest level + 1/2 his level = max. So I dunno are there restrictions or penalties? He said he was getting mad XP, I didnt notice a huge difference... Strange since i was always a EQ'er who eyeballed my XP bar like a hawk. I just enjoy the experience now of playing and am less concerned of the carrot infront of my face.