1) Be respectful - As I said, grammar and structure is collapsing nowadays. Be respectful, have a greeting and salutation, no "liek u shld tot b in RB & do da RBN", and treat them as the professional artist or manager they are.
2) Be clear - Tell them exactly what the Rock Band Network is and the possibilities could of taking part. Did you know there's been over 40 million downloads already and this was months ago that number was released?
3) Suggestions - Let them know they can do anything from their catalogue, but point out one or two you'd like and give a good reason why. That might help you get exactly what you want.
4) Redirect to an author - Give them the email address or Web site of an authoring company. RG Studios, Rhythm Authors, or even (especially) me. My address is [email protected] and I'm the owner of Ozone Entertainment if you would like to use me.
Hope that helps.