You need to pick one member of the family first.
How do you select which unit to attack with? Do you click it with a mouse or something?
Then I'm guessing you have an event like this:
On object PlayerUnitsFamily clicked
Inside this event the correct member (instance) of the family will be picked and PlayerUnitsFamily.damage will refer to this picked instance.
There are lots of other ways to pick instances, this is probably the most powerful and important concept in C2.