Extras roll as distinct groups and act on the same Action Card as usual. Divide each group's attacks up proportionately among the opposition, losing attacks against heroes who have Advantage over them. For example, nine wolves act on Seven and there are five player characters, so each hero is assigned two wolves. Any heroes who have higher cards than the wolves aren't attacked that round. (This keeps characters with low cards from getting attacked by every enemy in the chase.)
The opposite is not true. The player characters may always attack any foe or group of foes they have Advantage over.
Savage Worlds Deluxe Edition p. 95