I'm by no means a MAA expert. I've only fought 4 of the Group Bosses (GB), and I'm definitely not the strongest player on my allies list. But, I have come up with a few rules that I follow and it seems to work. It gives you a lot of Lockboxes (LB), but it helps your friends get a lot of LB too. Trust me, I tried my first time all by myself & it just doesn't work. You need other ppl supporting you, and vice versa.
First, you probably want some numbers right?
LB hero: Costs between 80 and 360 LB.
Personal experience: Costs for me range between 130 and 220 LB.
Baron Mordo : 50 LB; sofar.
Enchantress : Collected 481 LB.
Superior Spider-Man : Collected 497 LB.
Kang : ... Well, I'd just started.
With the absolute best luck, you can unlock a hero with 80 LB, with absolute worst luck it is 360 LB. Usually it's somewhere in between. For me it's been 130 with best luck, or 220 with worst luck. By just keeping these rules of etiquette in mind I was able to have like 260 extra LBs, which may or may not include fabulous prizes! Great, right? So here's the strategy I've been using.
My Basic Rules of Etiquette:
1: Don't Kill the Boss by Yourself! It's selfish because your friends get nothing. It's a waste of your UISO which you could use on the Spec Ops. Basically everybody loses. Don't do it!
*Note, it doesn't cost any UISO to attack an ally's GB. So attack as many as you can, just don't attack your own.
2: Wait to Collect your LB until your GB is Ready to Respawn! This gives your friends the absolute longest time to fight the boss too. They get a chance to fight the GB which helps them out, but it helps you out because everytime they win you get some LB. This means the difference of having a few friends give you some LBs, or most of your friends giving you LBs.
*Note, even if your friend Tony beats the GB, friends Bruce and Tasha are still able to fight the boss at 5% health. As long as you wait to collect your LB!
3: Invite everyone! If you try and select specific MAA players, you're limiting the number of ppl who will help you out to a very small number. And what if your friends Clint, Sam, and Peter are on busy, then you get nobody helping. But if you invite everyone, that's at least 50 ppl (maybe more depending on how many friend-players you have).
4: Login daily! If you want to maximize your LB potential, you need to login at least once a day. More than once if you have the time. This ensures that you're helping your ally's with their GB, so you can earn the most LBs. Also you can send out your GB as soon as it's ready to respawn to ensure you're getting the most fights during this limited time event. Also don't forget to collect the UISOs from daily gifts, and visit your allies for even more UISOs. But you knew that already, right?
5: Clean out your Allies! It's just like that movie Defriended only without anyone dying or some scary noise about Skype-ghosts. You can tell active players because they will have some kind of PVP rating and/or will have newer Heroes and Weapons on their Profile. If their last recruit is Blade, you might want to Remove the Ally and find a more active players, someone more likely to help with GBs.
6: Kill your Ally's GB..? This is kindof a moral grey area for me.. When fighting an ally's GB, you get a reward as long as you knock off 5% health. So really, you only need to do 5%. But, if you kill the GB you can get even more LB, plus a lot of silver. But it's kindof a selfish move, because you're taking all the silver so nobody else can have it. Or, depending on player strategy, you might be stealing it from your friend before they can get it. So should you kill your friend's GB, or leave them alive? It's up to you.
*As I mentioned before, even if Tony kills the GB then all the other friends still get a chance to fight at 5%.
So in Summary:
1: Don't kill the Boss by yourself.
2: Wait to collect LB until your GB is about to respawn.
3: Invite everyone.
4: Login daily.
5: Clean out old Allies.