User blog comment:FairyRaven/Fairy Tail Chapter 429 & 430: Code Blue & Purification Ritual, Review and Commentary/@comment-7418318-20150421105901/@comment-7418318-20150421112744

Sure they are all there, but just look at Briar, Jerome and Mary. They stand back in reserve and watch the samll-fry do the initial assault.

If the main cell is of a like mind, they too would stand back and wait until the ones we know of are defeated (or at least engaged and clearly not winning in a hurry) before they start to act.

Though there is also the possibility that despite Arlock's word the person in charge wants to sacrifice both the townspeople and the legions of small-fry worshippers for even more death to lure Zeref. If that's the case the main cell might not be among the throng of Avatar people we've seen so far. They could be over on a nearby hill to do the actual ritual for example.