I hate when people say that... Under the new xp system, a 6th player takes
a very small percent of xp from the original 5 members. Assuming the same
level, here is what happens:

Group5 gets 1000 monster xp
1000*1.14/5=228 xp per person (no factoring in multiplier for xp penalty,
which means nothing in this case)

Grou6 gets 1000 monster xp
1000*1.2/6=200 xp per person

Which means the new player needs to increase your kill rate a mere 14% to
balance out his presence in the group. Unless you are in an overcrowded
area, where you are fighting for kills, that should be no problem.