User blog comment:Thailog/Ep. 125 "Usual Suspects" discussion/@comment-5019916-20120414202553/@comment-1038387-20120414203137

Actually, Rocket hasn't appeared in an comic yet, and we're currently (as of issue 14) at September 6.

I thought it was pretty clear why she was added to the Team in this episode. Icon is with the League, so here, Rocket, you go sit in the corner with the other kiddies. If Plastic Man or Atom had sidekicks, they would probably join as well.