From the Reference Booklet

Team Battle
If more than one hero occupies a space that contains a monster, they can fight the monster together (the archer and heroes with bows can participate in the battle from an adjacent space). At the start of the current hero’s fight action, he invites the other heroes to participate in the battle.

Clarifcation on above: Adjacent space means archer is adjacent to the other hero (who is in the same space as the monster), correct?

Thanks!

I'd say "adjacent space" in this context means the archer is adjacent to the critter, not the other hero. That's the archer's special battle ability. Usually the other hero will be in the same space as the monster, but that is not the case if the other hero is also using a bow, then no one is in the same space as the monster. As long as both bowmen are adjacent to the monster it can be a team battle, even if the bowmen are not adjacent to each other.

I agree with the other answer. An archer needs to be adjacent to the critter to join in the battle. And it logically makes sense, if you think about it, as the archer can attack the critter solo if the other here weren't there so why couldn't the archer join forces with the hero in a similar capacity?

I feel that successful completion of the legends requires taking advantage of this capability of the archer to knock off several critters with efficiency.