You count as being Engaged if you are within melee range of the thing in question and are in combat
There's nothing RAW that says you can't contribute to outnumbering one foe even if you yourself are engaged with many other foes
There's not really a hard rule as to what counts as being engaged, so we have to apply some common sense here. Basically, if you're within range of attacking or being attacked by a melee combatant, you're considered "engaged" with them, and subject to the ganging up rule.
Let's take a quick look at the rules for fleeing from combat:
Disengage (Full Action)
You break off from melee combat and may move up to your Half Move
rate. Opponents whom you were engaged with do not gain the customary
free attacks. p. 192
And the rules for ganging up:
You have an advantage when you and your allies engage the same foe in
melee combat. [...] p. 198
So we've established that you have to be in melee combat in order to benefit from the gang up rules. I propose 3 guidelines to determine if you are indeed threatening an opponent:
- Can I smack the enemy?
- If I were to run, would they get to smack me?
- Is it trying to kill me?
Keep in mind that Dark Heresy is a very abstract game, so if it makes sense, run with it.