that is a part of the raiding group
I take the liberty to read this as "the one that is closer" ...
Since as it was stated multiple times before, you dont know who is or isnt "part of the raiding group".

People just dont have signs "red health = enemy" and "green health = aly" abowe their heads. :-/
I would not waste it trying to hit the group that just showed up and are flanking the goblins attacking me.
Well ... they showed simultaneously, at best few seconds difference ...
Wich was also stated before ...
And as stated before, on the same hill are another goblins who (
as we as players do, but our curent example character, that NPC Tiefling we are talking about now, dont know) actualy ARE part of "the raiding group attacking your comrades".

I wonder why you keep ignoring crucial parts of default setting. :-/
Based on how the game presents it the player party is immediately hostile to the goblin raiders when they approach.
As stated before, we are not talking about player party ... point of this topic is actions of NPCs towards our player party ... so who is or isnt hostile for player party is irellevant. -_-
They aren't even in range of most of the defenders until they've moved closer to engage the raiders.
Indeed? O_o
I would swear that i was able to attack Zevlor (for sure i even killed him few times) from my original position ... but maybe i remember this wrong, or there was some changes then.
And it'd be a fool who turned around to start attacking someone who just showed up and were attacking the people trying to kill them.
I repeat again:
1) They appear at same time.
2) They didnt attak anyone yet.
What is so hard about this concept? O_o
Turns exists ... PC party is not first ... you (the NPC Tiefling) are first ... you see Goblins:
3 of them are on the ground, with some Bugbear, and one (or two?) Worgs ...
3 of them are on the hill, next to one of those on the hill also are Githyanki, clearly a Shar worshipper and some really weird looking Elf.
This is your situation ... and i repeat (for like 6th time), you are first in turn order, no shots were fired, no hostility was stated, nobody expresed any aeligance to anyone else.
Comprende?
You decided to attack Goblin that is close to Aradin ... that is fine, its your decision ... you say you would be a fool to attack someone else ...
Well ... next turn one of those Goblins on the hill casts Witch Bolt on Aradin and kills him (lets say it was crit) ... still feeling smart? O_o
I mean ... you can, after all you cant kill everyone and one of enemies would attack him anyway, this is just tough luck ... but ... i wouldnt.
And yes, i
purposefully didnt say wich one attacked him.
Wanna know why?
Bcs it dont freaking matter!
Even if it would be our GoblinTav, game still dont recognize it ... that is btw, another problem im complaining about ... but lets stay focused on first thing first.

I'm sorry, but I can't imagine a scenario where I'd want to kill someone killing my enemies purely based on how they look.
Thats bcs you dont listen ... for 7th time (yes, i will be counting it from now on) ... nobody killed, nor attacked anyone yet.
As long as you keep adding your own valuables, you change scenario ... as long as you keep changing scenario, you dont work with default one ... and therefore, logicaly, you can understand it.
Right now i only wonder if you even realize it, or do it on purpose.
And yes, you have to decide "purely on how they look" ... bcs that is all info you have.
If I turn around and attack them immediately I risk being overrun by two forces simultaneously for no reason.
Sigh ...
How do you know they are two forces?
(
Hint: You dont!)
You can indeed presume they are not together based on his followers ... i can accept that.
Presuming our PC followers seems trustworthy (meaning only Gale ... maaaaaaaaaaaybe Astarion, if we really want to be nice) ...
And presuming our PC even have any followers in the first place.

But lets put theese conditions aside, stated example was Lae'zel, Astarion, Shadowheart and GoblinTav ... lets stick to that.
As for when to confirm their intentions? The very second they make a hostile action they reveal their intentions.
See this is what im talking about, you again are changing parameters.
(Gods im glad we are not working together, we would drive each other to madness. :D)
For 8th time ... no intentions were revealed, you are F-I-R-S-T in order ...
And you have to decide NOW ... that is how things work in this game.
Anything that happens after you END your turn, is irellevant for THIS example, bcs then situation will change.
Frankly if I am in the position of Zevlor I don't care too much if these people ARE an evil adventuring group. They just killed my enemies and seem willing to keep killing them.
Wich is once aggain completely different situation, but if you want to talk about this aswell ...
Yes, some gratiture makes sense ...
On the other hand tho, universal friendly welcome from whole grove seems a bit extreme ...
We are clearly different people, but if i would be in the position of Zevlor, i would still require some ensurances from them ... as also stated abowe, things like surendering your weapons, warning that guards will keep extra eye on them, and only if they promise to behave and will seem thrustworthy (persuation check) ... i would be willing to allow them to enter.
Its called being carefull.
I don't have any good options left because the druids have gone crazy and the last group of adventurers I hired ended up unable to get the job done.
I dont know what group of adventurers you are talking about.
What should I do? Attack my last chance to maybe save my people because they look suspicious?
Depends on wich scenario are we talking about.
The one i presented? They just appeared on the hill?
Then yes, you should attack them ... bcs at that point they are not last chance for your people.
Or the one you keep replacing it with? They attacked another Goblin?
Then no, you should not attack them ... enemy of my enemy is, well at least usefull tool ... you should still be carefull tho, politely and carefully explain to them your situation, ask them to submit your conditions, and only(!!!) if they show they are willing to help, try to push it even futher.
You do not make an enemy of a bystander
Its getting old allready, doenst it? -_-
Sigh ...
Okey, how do you know they are "a bystander"?
(
Hint: You dont!)
Option B will almost definitely get your people killed, while option A only might based on an assumption of the second group's intentions.
That seems like overstatement ...
Are you taking under concideration relative damage potential difference between Fighter with 1h ... and Wizard?
Not to mention that the second group is on the other side of the raiders. Your melee guys will be getting shredded without cover fire while you try to snipe across the map at people who may have no cause to attack you.
I dont see what are you trying to say here. O_o
Plus the way the fight is set up to take the hill they need to have already started attacking a few of the raider goblins, so you immediately know they aren't on Team Absolute.
What? O_o
Why would they need to attack anyone to start on the hill? o_O
You can start there quite easily.
When the group first appears and have made no move to say who they are or what they intend
Exactly.
there's no advantage in attacking them.
True ...
Except disabling potentialy hostile caster that can cause much more problems than some lowly, rusty-scimitar wawing Goblins.

Once they DO move they are attacking the goblin raiders so I know they are enemies of my enemies and will act accordingly.
You mean IF they attack Goblin raiders ...
See?
You still are taking aeliganges under concideration. :-/
The one who did not arrive with the other goblins and is standing with a bunch of non-goblins is the one I wouldn't immediately try to kill.
Okey.
I gues everyone should have right to make his own misstakes.

If you're asking how I don't get it confused with the attacking goblins once battle is joined and things get messy
Eh ... nope, that was not my question.
tl;dr: I wouldn't attack a second group that just stood there watching the battle, as that turns them into an enemy regardless of what they intended to do. I also wouldn't attack the second group if I saw them attacking my enemy, as they are killing my enemy for me and that directly benefits me. If they make a hostile move toward my guys THAT is when I'll respond in kind, because at that point they have demonstrated their intent to do me harm.
Aaaand that was not my question either.

But i gues thats the best i get. :-/