This achievement is actually very easy.
The only problem you're likely to encounter is to get a 4 player band together over Live. Absolutely nobody wants to sing. So, a simple solution for this is to play 2 parts on your account. Play one of the instruments yourself (guitar, bass, drums, whichever you prefer) then put your mic up to the speaker on your TV/sound system.
Play medium or higher (the higher the better) on your instrument and set the mic to beginner. Then all you need to concentrate on is performing well on your instrument because the mic will take care of itself and generate tons of star power for you in the process.
I played this in a 4 player band with myself on hard bass and beginner mic, and the other players on hard drums and expert guitar. We ended up with a score of over 1,000,000.