This can be done with as few as 3 people, each on a different instrument (no duplicate instruments allowed). Make sure everyone is on Medium difficulty or higher, and choose the song "2112 Part 4" from the Quickplay+ menu. Complete the song with any amount of stars, and this achievement is yours.
Edit: If you're going to vote negatively, PLEASE leave a comment explaining why and what you think should be added or taken out of the solution. I write these to help others in the TA community, so if you would contribute also (clicking the thumbs down button is not contributing) that would be great. Thanks!