Easiest way to answer this is to tweak the question:
- If the first PC is trying to hide (not very well) and the group of enemies notices him and moves forward to engage... then another PC jumps up to attack the group from hiding, is the group surprised?
I think most of us would probably answer 'No'. Once the group saw the PC (through Passive Perception or whatever), then initiative would be rolled. And in the first round of battle, it's only those creatures that did not see a threat at all that are Surprised for that first round. It doesn't matter if a creature can see ALL the threats, they only need to see one to not be considered Surprised.
The Minor illusion scenario is the same exact thing. As far as the group is concerned, the illusory PC is a threat and they move to attack it, with everyone rolling initiative. Since they noticed what they thought was a threat, they get to act in that first round of combat. As of course does the real PC, because he also noticed the threat-- the group approaching the illusion. So neither the group nor the PC are surprised, and thus they both get to act in the first round. And just to add to that... the real PC that was hiding does get to make his attack with Advantage because he was successfully hiding from the group.
So the Minor Illusion trick is actually not that worthwhile if the PC is trying to gain Surprise. He really should just Hide and hope his DEX (Stealth) check is higher than the group's Passive Perception. The real reason to use the Minor Illusion trick would be to draw the group away from some location, as they would obviously move to where the illusion was (until of course they discovered the illusion and came running back.)