If you want to take a quick look at my lowbie arc to see how it plays (1345), I did something similar, but I used an ally rescue instead of an ambush. If you set the group guarding the "kidnap victim" as allies, they'll be, well, allies. In my arc they spawn when the boss is defeated, but you can just as easily make them spawn when the boss fight just starts, or have both the allies and the boss spawn off a third objective in the same room.
With careful selection of the map and making sure you reserve the "back" spawn points for this, you can give the player whatever reinforcements you wanted for that boss fight. If you make both boss and allies spawn off a glowie, you'd need a back room with 3 placeable objectives in close proximity, if one of them is a glowie it can work - in fact, my arc has both the allies, boss, and a destructible object always in the same room as far as I can tell.
The important thing to remember is that having an ally guarded by allied mobs will make it impossible to "rescue" said ally, so don't make it a mandatory objective. Also, this will allow you to get more lines in. Finally, the ally being "guarded" will not lift a finger during the fight, but the guards will. Said ally may actually join with you if all the guards are defeated (I haven't checked).
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Known issue unfortunately. Ally Ambushes don't work simply because only one of the Allies will actually run to where they should.
That's a very interesting idea, Aliana, I may have to test a few maps to see if I can do that. Unfortunately it's an abandoned-warehouse map, and those are always pretty generous with the "Back" spawn points. But on the other hand, it would allow me to assign dialog specifically to one character (the 'boss'), which is a big advantage given the structure of the mission.
Thanks! And thanks, Destructor, for your response too. Hopefully it'll get resolved soon!
It would be a neat idea if it worked.
But you can only do this for captives, not for allies.
The difference being that the captive turns hostile when you make its guards allied.
An Ally always needs hostile gurads, you cannot even give him no guards, not even when freeing him is not a mission goal, as it seems.
I'm actually pretty stuck myself with trying to do a mission where in the end a lot of good and bad guys just stand around and wait for you to attack one of the bad guys, then all fight.
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An Ally always needs hostile gurads, you cannot even give him no guards, not even when freeing him is not a mission goal, as it seems.
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You can give an Ally no guards - set the guard spawn size to "single".
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I saw that happen today too when I porbed the MA some more and found that neat for my use, but thought it was rather meant to only spawn a single enemy and it scaled down to zero because there were no minions in that custom group and I was solo or some.
Does that actually mean no guards whatsoever? That'd be cool. They should call it "None" then though. 
I have to agree on the label. "Single" made it sound like one guard, and I didn't know differently until I read it on the boards a while back.
Dec out.
I found that spawning allies with triggers makes no guards.
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With the setting on what? Because I've absolutely found that not to be true as a generality.
Dec out.
Has there been any more testing of this? I want to have a group of reinforcements enter the mission for the final showdown, but I don't want to knock myself out trying if it's already been determined it doesn't work.
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I believe it should work. They should spawn in a good ways back then run to where the player was when he triggered the spawn. I haven't fully tested this yet but that what it seemed it would do.
Actually I just tried this some and only one of the spawns would rush in to help the rest just stayed where they was. A patrol may work best if you know where it will spawn. Hopefully they will address things like this to add more immersion into the MA arcs
So I have an arc (shameless plug! "One Million Eyes", Arc ID # 71933) that has been getting a lot of really kind, favorable reviews. However, one recurring comment is that the ambush of allies in the final mission is broken.
Essentially, 2 of every 3 critters in the ambush remain at the spawn point and don't do anything.
This ambush is necessary for two reasons: to give people help with the final AV/EB if they need it, and it's thematically perfect. So I can't do without it. And I can't seem to get it to spawn any closer to the AV, even though they're both set to "Back" placement.
So one workaround I've found is to pull the AV into the hallway. This seems to bring it close enough to trigger the entire friendly ambush's aggro range, or it brings it close enough so that the pathing isn't all wonky. Whatever the case, it works.
But, it's a bit of an imposition on the player, who may not necessarily be the "pulling" type, or appreciate having to put in the extra effort (especially if they can easily handle the AV alone).
So, is there any way around this? Commenters have recommended that I "fix" it, but I've tried various things and nothing seems to work. It doesn't seem to be dependent on factors such as powersets (I thought some of the allies might be switching on their toggles and then 'forgetting' to move), and as I said placement doesn't seem to make a difference.
Thanks in advance!