You can either buy all characters legit or there is a cheat way which is to enter the 'Unlock everything' cheat from the cheats menu in options.Must be done with a guitar
Hold the 4 corresponding frets (Colours showing in the picture) and strum after each one. If you enter it right there will be a cheer and a pop up of 'unlocked everything'
Press start once you are finished, then head into a career and the store and this along with the other 3 buying achievements will unlock.
Here's a video to go with the picture.