LEGO Rock Band

Here are the songs needed to play for each entourage member to unlock so you can buy them. (Credit for list goes to Son of Kyuss @ xbox360achievements.org):

Camper Van > LEGO City Station > Grace by Supergrass
Minibus > Brickton Manor > Girls and Boys by Good Charlotte
Speedboat > The Jolly Rocker > Accidentally in Love by Counting Crows
Chopper > LEGO City Speedway > Make Me Smile (Come Up and See Me) by Steve Harley & Cockney Rebel
Tour Bus > Stud Farm > Valerie by The Zutons
Hovercraft > The Bone Cave > Kung Fu Fighting by Carl Douglas
Submarine > The Abyss > Naïve by The Kooks
Private Jet > Brickingham Castle > Two Princes by Spin Doctors
Teleporter > UFO > Fire by Jimi Hendrix

Entourage members are unlocked during the World Tour mode. After you finish a gig, after the song results screen, you'll get a notice of new items you've unlocked. Entourage members are various different staffers or professionals (like a Music Producer, or Marketing Exec, or Stylist) and are listed as being "Available in the Office".

Once you unlock an Entourage member, simply exit out to your Rock Den, and enter the Office. Scroll to the left, and you'll be at the Entourage section, where you can purchase Entourage members you've unlocked.

There are 10 Entourage members total in the game. The achievement will unlock once you've hired/purchased the last one. Your Career Stats screen in the Office keeps track of how many members you've gotten so far.

It IS possible to miss one or more Entourage members going through the tour. All the Entourage members are unlocked after single-song gigs. If you've finished all the Rock Power challenges and still don't have all Entourage members, just go back through the venues and play any single-son gigs you might have skipped your first time through.

Just adding to Camdon ZN's excellent guide, here's the list of entourage members.

Increase fans:
Street Team Boss
Music Video Director
Stylist
Spiritual Guide
Pro Sound Engineer

Increase studs:
Intern
Record Producer
Marketing Executive
Accountant
Top Promoter