To get a "unison bonus" all band members must hit the Beatlemania Phrase (also known as Star Power Phrase to the Guitar Hero fan or the Overdrive Phrase to the Rockband fan), and in this case you'll only need the drummer, bassist, and guitarist to hit each of their phrases respectively. Once you do this you'll gain your normal 1/4 increase of your meter as well as an extra 1/4 if you score on the "unison bonus."
Now the songs you'll need to perform this somewhat frustrating task (as it seems someone always messes up at the worst time) are:
1. And Your Bird Can Sing
2. Birthday
3. Hello Goodbye
4. Revolution
5. Taxman
6. Yellow Submarine
Good Luck!!
Also remember that in pretty much every song there will be multiple opportunities to hit these "unison bonuses." And if you find you and your band having a rough time nailing this one down, you can always go to a lower difficulty, but be careful not to go too low or people will lose their focus because of how easy it is. Play at a level you're comfortable with and you'll have this baby in no time.