If you can tag AND kill fast with the lowbies in a group and the level 90 out of the group (to help kill faster but make sure it does not tag) then that's the best. Yes the level 90 will reduce the exp in the group but if the mobs are not grey to the 90 then you don't lose a ton, but you will lose depending on how many in the group, if you have the 90 and a lowbie in the group then you lose 50 percent like I was. If you tag with the lowbies and help kill with the 90 out of group and the lowbies don't die then go with that, otherwise you have to bring the tank into the group to tag and take agro.