The Camp Rock Movie and their teenage pop music make them sought after superstars. Nick, Kevin and Joe Jonas know how must of their success is because of their fans and they always give out the autographs upon request.
Meet them before the concert. All over the web you find pictures of these gentleman thanking their fans by signing autographs just before the concert (just outside the doors.) Arrive 30 to 60 minutes before the show and start asking security where the line is for this opportunity.
Meet them after the concert. Radio stations and other local venues always have contests in meeting this trio.
See them at their hotel. The band is notorious to check in under the name Brian Wilkerson. Never would I suggest harassing superstars as that is against the law, but there is no harm in seeing if you can catch an autograph in the lobby with permission from the hotel manager. Many fan sites suggest this is a popular way to get signatures.
Write them a personal letter and ask for their autograph. Send your letter to:
Jonas Brothers
c/o Disney Channel
3800 Alameda Ave.
Burbank, CA 91505