Jennifer Aniston has been getting into the groupie spirit by joining John Mayer on his tour bus for the US leg of his summer road show.

Mayer brought his musical wonderland to 2008 Milwaukee Summerfest at the Marcus Amphitheater on Wednesday night - with Aniston in tow of course.

"They were on John's tour bus hanging out most of the time before the show," the event's publicist, Kristi Chuckel, told E! News.

"She was backstage standing in the wings, hanging out and watching the show."

Mayer's Number One Fan recently followed her man to the UK, where she reportedly had potential love rival Kimberly Stewart ejected from the backstage area of his Hyde Park, London concert.