It's possible for Space Wolf units to have models with different armour saves. So the normal casualty removal and armour save rules are altered slightly.
The opposing player rolls to hit and wound for whichever unit he is firing with as normal. However, when the Space Wolves player makes his armour saves, he uses the save for the armour worn by the greatest number of models in the unit. In the case of a tie, the Space Wolves player may decide which save is used. Any casualties removed after saving throws have been taken must come first of all from the models whose saving throw was used (even if hit by weapons that don't allow a save to be taken, like power weapons etc.).
For example, if a Wolf Guard bodyguard consisted of three models with Terminator armour and two with power armour then the Terminator armour save would be used, but casualties would be taken on the Terminators first of all.
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): A character accompanied by Fenrisian Wolves is a Mixed Armour unit, as is an Iron Priest accompanied by a Thrall bodyguard.