This can be done with as few as two people, but more people make it far easier. You can choose a long song like Heard it Through the Grapevine (DLC) and have an easier time of this, but this can also be accomplished with only two people on Free Bird if they can play almost perfectly on Expert Drums and Guitar or Bass. I did this playing very well on Expert Bass while singing with a mic stand on Expert Vocals, while my partner gave a gold star performance on Expert Drums. If you get four people for this, not everyone needs to play on Expert as the 8x multiplier from having everyone in overdrive at the same time provides a massive boost, so be aware of that. You can find a group in the Achievement Trading Thread here.