Skyhunter Strike Force

Flying
Melee (Whenever this creature attacks, it gets +1/+1 until end of turn for each opponent you attacked this combat.)
Lieutenant — As long as you control your commander, other creatures you control have melee.

The List

Phyrexia: All Will Be One Commander

Flying, melee
Lieutenant — As long as you control your commander, other creatures you control have melee.

Creatures that enter the battlefield attacking were never declared as attackers, so they won't count toward melee's effect. Similarly, if a creature with melee enters the battlefield attacking, melee won't trigger.

It doesn't matter how many creatures you attacked a player with, only that you attacked a player with at least one creature. For example, if you attack one player with Skyhunter Strike Force and another player with five creatures, Skyhunter Strike Force will get +2/+2 until end of turn.

Melee will trigger if the creature with melee attacks a planeswalker. However, the effect counts only opponents (and not planeswalkers) that you attacked with a creature when determining the bonus.

You determine the size of the bonus as the melee ability resolves. Count each opponent that you attacked with one or more creatures. It doesn't matter if the attacking creatures are still attacking or even if they are still on the battlefield. It also doesn't matter if the opponent you attacked is still in the game.