You'll need to play in Band Tour mode and get your cumulative score over 10 million for this achievement to unlock. This score is sum of the highest scores you've gotten for each individual song with your band, so no simply replaying one song over and over to accrue points that way, sadly. The cumulative score is on display right at the bottom of the song selection screen, so it's easy to know just how close you are to getting this. You can play any given song with any combination of instruments and difficulties; the highest score you get with any configuration is the one that will count.
Generally speaking, so long as they're not failing out, the more players you can get into the mix, the merrier. More note charts on the screen make for more points available in each song, after all. If you can talk a roommate into limping through some songs on Medium bass or get a sibling to sing along with you on Easy, that'll really help matters. (Two really talented players who play absolutely lights-out can probably get this achievement by themselves... but more band members will really make things tons easier.)
As with all Rock Band games, keeping combo and getting unison bonuses on overdrive phrases are supremely important towards getting a good score. I know it might make your pride sting some... but playing one difficulty level easier than you usually do can really be a big boon, especially on more difficult songs. (Hitting 95% on Expert often nets you fewer points than hitting 99% on Hard.) You should also take care to make sure that band members have overdrive activated at the same time whenever possible; that increases the multiplier for the whole band, which can really ramp up one's score in a hurry.
Us, we did this achievement with a three-person band (guitar, drums, bass) of reasonably experienced people, all playing on Expert in the early songs and dropping down to Hard later on in the setlist, and hit 10 million with multiple songs to spare. A four-person band would've gotten us there even earlier. If they're hitting unison bonuses and using overdrive in a smart fashion, a four-person all-Medium band could certainly achieve this as well. (Heck, maybe even an all-Easy band, too. Wouldn't surprise me if they could.)