In Hammer of the Emperor it states

"A Player who has multiple Comrades can issue orders to each one, but can only give the Ranged Volley Order and Close Quarters Order to one Comrade each turn." Page 63

This seems pretty straight forward but in the Lasgun Volley talent it states

"For each comrade who is part of any Ranged Volley Order this character issues, this character deals +1 additional damage(to a maximum of +3) with any attack he makes with a las weapon...." Page 100

When it says up to a maximum of +3 damage it seems to imply that you can have multiple comrades participating in the Ranged Volley order, although it previously says you cannot. Am I misreading the rules or is this a mistake in the book?

Well, either it might mean that, since the Order is a half action, you can only issue it once, regardless of actions, or number of Comrades, or you can't order it to yours, AND one of your teammate's comrades, id you have Sweeping Orders, I don't know. Since you have multiple Comrades, I assume they just wanted to make sure you didn't try to issue it twice, upping the already improved bonus. Beyond that, I'm not certain, either.

I think RAI for Lasgun Volley was that you get a +1 Damage for issuing a Ranged Volley Order to your Comrade. Ranged Volley Order isn't available as a Sweeping Order, and the same book clearly states that Comrades can't be given the same Order. Otherwise the Commander gets +25 to hit and +3 damage for free, if he has all five comrades.

Back Them Up! functions as a workaround order to solve that very problem, enabling them to get +25 to their BS. The +3 damage is debatable, but I roll with it, seeing as the +25-30 BS is a thing.

That is true. I wonder why they allow the Ranged Volley and Close Quarters to benefit all the players, except the player who actually bought to comrade. I wonder if that is what the designers were originally thinking for the commander advanced specialty; multiple comrades give a +5 BS/WS bonus to the Ranged Volley/Close Quarters but multiple comrades don't actually do the order, only one. That way you get the BS/WS bonus for multiple comrades all firing, but you don't get to use the Get Them order for each one, preventing the +20 damage would be able to do per shot. You would have to get the Lasgun Volley talent to increase damage per shot, up to the max of +3 damage.

You can give Back Them Up! and Get Them! in the same turn, I'm pretty sure. Having 5 comrades means you can buff one person massively, or everyone a little bit extra, however, Get Them! doesn't stack for multiple comrades. Personally, though, as a Commander, I usually assign my extra comrades to characters like Commissars and Stormtroopers.

I think the way it looks is that the Commander himself wouldn't get any benefits from having the multiple comrades, it is designed more to let him/her boost the rest of the squad.

It is quite an investment though, costing 800xp to assign the first comrade to a squad mate. Although attaching a ogryn to a brawler could be quite a boost with the +10 WS and additional wound state. Personally I think it makes more sense to invest in other things like Radiant Presence or Dig In before buying comrades though.