I have taken a great deal of time to read the rules very carefully and i have prepared this to help quell this dispute.
I had also heard from a GW employee that if a unit joins an assault the defender cannot direct attacks at them. This, as i have found is only partially untrue.
Take this example: (For the sake of everyone's sanity bear with it as i have used the entire assault phase in my demonstration. Please, read it all)
khorne berzerkers are in combat with the thousand sons at the start of the assault phase (this is NOT combat). The unit of terminators wishes to assault them.
As per the rules, the closest model in the Squad moves up to 6" to make base contact with the enemy models.
The closest terminator has now made his assault move. The rest of the terminators may now make thier assault moves in any order.
This terminator cannot reach the berzerkers on the far side, and thus must attempt to make contact with the enemy elsewhere.
The terminator is well within assault distance of this berzerker
The terminator now moves into base contact with the enemy.
The last terminator, which obviously able to reach combat, cannot get into base contact, and thus most move within 2" of another model in his unit that is in base contact with the enemy.
all terminators have been moved, and no enemy models are free to react, thus all models are now "engaged". This is the beginning of combat.
(After all assault moves, but before blows are struck)
Now onto the root of the problem...
This is where all the confusion lies. The key lies in the term in the rulebook. "Models" p. 41 BRB
"Models that were engaged with just one of the enemy units at the beginning of combat (before any model attacked) must attack that unit."
If we take a look at the numbered khorne berzerkers in the picture we can see this.
Berzerker 1 - base contact with tsons (He must attack them)
Berserker 2 - Base contact with tsons (he must attack them)
Berserker 3 - Base contact with tsons (he must attack them)
Berserker 4 - Base contact with both units. (he may choose which unit to attack)
Berserker 5 - Base Contact with both units. (he may choose which unit to attack)
You look at each model in the defending unit and for each one do the following...
Check for base contact. If the model is in base contact with an enemy model it must direct it's attacks at the model in base contact.
If that model is not in base contact with an enemy it must check to see if there is a a friendly model from the same unit within 2" that is in base contact with an enemy model. If there is, it may direct it's attacks and any models in base contact with that friendly model.
If not, it cannot attack at all.
In multiple unit combat, you tally your attacks model vs. model. Then roll your dice as a unit.
I hope this helps.