The Almandinger kid stated it was "D.J." or Dominic Johnson that pulled the trigger. He named one of the others in another instance, according to a previous article..i think it was austin bennet they said. 

All that were there were guilty of taking his life. Any one of them could have stood up and said "this is enough" but then again..teenager cliques can form a very destructive path in a millisecond. 

I remember as a teenager a second group of my friends invited me over to hangout...where i was accused of stealing an n64 game (even though i didnt even own an n64), 8 of them were ready to put me in the dirt...the main accuser was the one who inevitably took it. Only one person  prevented them from jumping me which was all that was needed. Needless to say, i stuck with my 1st group of trustworthy and respectable friends from then on. 

So I think the group as a whole is guilty of the killing.  Because it only takes one person on the other side to say "stop" which never happened here.