Are they Visible or Not?

[Compass Rose]

I have a visibility game question.

Say you have two squads sharing the same position and both have LOS on the enemy and the enemy has LOS on the hex they occupy. If squad one opens up fire, and the other squad, (squad two) has either not fired at all and/or has a "Hold Fire" command against it, is squad two visible to the enemy also, or are they not visible until they open up fire, and just squad one is visible to the enemy?

Also, if only part of a squad opens up, are they the only ones from the squad to be seen by the enemy or does the full squad become visible to the enemy. even though some of them have not fired yet?

Thanks