[R] Getting the groupmean for each person

[Felix Eschenburg]

Hello list !

I have a huge data.frame with several variables observed on about 3000 
persons. For every person (row) there is variable called GROUP which indices 
the group the person belongs to. There is also another variable AV for each 
person. Now i want to create a new variable which holds the group mean of AV 
as a value for each person.
With tapply(AV,GROUP,mean) i get the means for each level of GROUP, but i 
cannot find out, how to give every person the groupmean as a value (every 
person should have the same value as every other person in the same group). 

Has anybody any ideas how to do that ?

Yours sincerly
Felix Eschenburg