Strongarms do Stack, At lease to my previous knowledge. I will try to explain it has simple as I know it.
If one person is using Strongarms and uses a knockback ability, everyone in the group gets the one Strongarm debuff on thos enemies.
If two-four players are WEARING Strongarms, The Debuff can stack multiple times in damage. But Each person WEARING the Strongarms has to individually do a knockback ability on the enemies for it to stack multiple times.
Hope this helps guys. My information could be wrong, but I am pretty positive this is how the Strongarms debuff works in a group.