Jennifer Aniston has resorted to taking the bus these days, and it has nothing to do with the summer's sky-high gas prices.

E! News has learned that actress is not just attending boyfriend John Mayer's concerts. She's gone groupie, joining him on his tour bus.

Mayer kicked off the U.S. leg of his summer road show Wednesday night, with Aniston by his side backstage. The couple arrived together at the 2008 Milwaukee Summerfest at the Marcus Amphitheater on his rig.

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

Mayer went on at 9 p.m. and played for about two hours. All the while, his No. 1 band aide was watching from the side.

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

And it looks like she was enjoying herself.

Karra Barnes, a Mayer fan who also saw Aniston backstage, tells E! News, "She seemed pretty into it. She supports him greatly, you can tell.

"John Mayer also seems to be playing with a lot more passion. I saw him last year at Summerfest, and he was very passionate then, but now something seems a little different about him."

Perhaps it has something to do with his ride share?

Mayer, presumably with his Friend in tow, plays tonight at the Verizon Wireless Amphitheater in Maryland Heights, Mo.