I am no authority on Pack Instincts as I do not use them deliberately.
My understanding is that each additional member, with the same Pact Instinct will increase the amount of damage done by every member of the pact.
As an example that is not based on any known by me reality.
Mob A is of a Pact Instinct and does, for simplicity sake, 20 damage, when another member is added then each members damage is increased by say 20% or now become 24, 3 = 29(roundup), 4 = 35 (roundup), 5 = 42. So the pack may be doing 210 damage per 5 seconds.
I have never seen anything authoritatively when Magic Attacks are involved, i.e. does the damage done by any particular spell increase the same way the melee damage does or is it purely melee damage that is increased.