From Ganging Up, page 101, Savage Worlds Adventure Edition:
Ganging up allows attackers to flank, exploit openings, and generally harass a foe. Each additional adjacent foe (who isn’t Stunned) adds +1 to all the attackers’ Fighting rolls, up to a maximum of +4. If three goblins attack a single hero, for example, each of the three goblins add+2 to their Fighting rolls.
All capable characters adjacent and hostile to the victim count for Ganging Up. It doesn't matter if those characters are Wild Cards or Extras, if they are player characters or non-player characters, if they are summoned via the summon ally power.
The only concerns are capacity, adjacency, and hostility to the victim of the attack.