I disagree, the word surprise has a meaning... to not see something coming, to be taken by surprise... the game ADDs 'if in the first round of combat.... no where does page 189 (the only surprise rules I can find) say "Ignore what the word surprise means in every day language and instead make it a term that only ever means in the first round of combat"... so yes, you can surprise someone when you do something surprising...
I really don't understand how it is possible to read the word surprise, and not think it means surprise...
Just like when someone said person A can't surprise you if you know person B is there.... or to put this in a very easy to understand analogy, if my girlfriend walks up to you and says "I need help I'm lost, can you show me on my phone's map where we are?" and I am hiding near by, when you look at the phone to help I can't jump out and surprise you with a knife... after all you knew she was there, she isn't surprising you... and according to you guys if two people are working togather they can't set up for one to surprise the target unless they both do...
This may be pointless to discuss, because if you rule that the "surprise" required by Assassinate is no more and no less than the general English-language dictionary definition, this all comes down to you, your Websters, and the psychology of the opponent in question. There's not a whole lot for me to explore there with you.
But, because I honestly think it's interesting, let's explore the situation above, with the understanding that the "surprise" of Assassinate is the same "surprise" from page 69 of the Basic Rules.
Surprise, on page 69, and, as I extrapolate, to other places it is referenced with regards to D&D 5e combat, is all about
the awareness of potential combatants. If I am unaware of a group of potentially hostile creatures, when they jump out of hiding, burst through the door, drop in from the skylight, I am surprised and unable to take normal actions within the first 6 seconds of their assault. (Though, as discussed at length, I am able to take reactions at a certain point, depending on my initiative roll.) What this means, broadly, is that, as long as I am aware of
any combatants, I am ready for combat. On page 60, the basic rules state, "In combat, most creatures stay alert for signs of danger all around." In other words, once I am ready for combat with one person, I am also expecting combat from some other direction. I am able to act normally, though I might not be aware of some hidden combatants, and so am unlikely to target them with my actions. So, if a hostile creature approaches me in plain sight, while another hostile creature slinks along, a bit to the side, or a bit to the back, I am ready for combat the moment I see that first hostile creature. Learning about the second hostile creature doesn't surprise me.
On the other hand, what if I am aware of someone, but do not believe them to be a potential combatant? If that is the case, sure, I could be surprised. I'm not aware of the chance of combat. The woman standing with a map might give me no reason to suspect that any hostilities might occur, so when you jump out from behind the trash can where you are hiding with a knife, I am ambushed and surprised.
For this to work, however, I
truly have to be convinced that your girlfriend is not threatening in any way, which, as Hriston says, depends on the setting. Are we walking down Broadway in the middle of the day? Or are we walking down a street in my neighborhood in the middle of the night? Does your girlfriend have any weapons or other signs of combat on her? Does your girlfriend look like somebody who would normally set foot in my part of the world? And ask me for directions? (Like, you know, am I a goblin and she's a human?) As long as we can reasonably establish that I am completely unthreatened by the creature I'm aware of, and, of course, unaware of all the nasty adventurers with knives hanging out behind a trash can, I think we can say that I am surprised, with all the benefits that entails. (I don't act normally during this round of combat, I could be assassinated, etc.) I'd also rule that your girlfriend is also surprised, because for her to act in more close coordination with you would probably give away the ruse. So she can't act normally during the first round of combat either. Though some really good rolling on her part might change my mind.
Oh, and if, despite all of this, I, the goblin, decide to attack your girlfriend with a shortsword instead of helpfully giving her directions, I'm aware of combat (since I started it), and am not surprised.
Those are the criteria that I'd use. I actually think we had a situation somewhat like that in my game once. (The bard approached a potentially hostile individual, and tried to offer his services as a mook. The two talked, the bard succeeded on a number of persuasion and deception rolls, and then the rogue jumped out from behind a tree and shot an arrow in the hostile individual's face.)