See pictures of Nicole Richie joining Britney Spears on stage -- giving her a big hug and crawling on all fours -- at her Feb. 18 Las Vegas show. Credit: Courtesy of Nicole Richie

Slave for Britney Spears! Nicole Richie joined the pop singer on stage during her Tuesday, Feb. 18 concert at Las Vegas' Planet Hollywood. The 32-year-old fashion designer, and Spears superfan, was pulled up on stage during a raunchy performance of the singer's "Freak Show."

Richie shared photos on Instagram from the wild night that included her crawling like a dog on stage while Spears walked her on a leash. In one snapshot, Richie gives Spears, 32, a big hug while surrounded by her scantily-clad male backup dancers. She captioned the embrace: "#GOODBYE." In a second picture, Richie is seen laughing while holding up a large T-shirt with Spears' face on it.

"Can we all give a round of applause to @NicoleRichie for playing along in the show tonight?" Spears tweeted early in the morning on Feb. 19. "U looked hot crawling on all fours."

The "I'm a Slave 4 U" singer also tweeted Richie directly: "@NicoleRichie Thanks for dancing tonight! xoxox." Richie replied back: "Am I alive?"

This isn't the first time Spears had made a celebrity join her on stage and crawl on all fours. Mario Lopez and Lance Bass had the honor last year, as well as Spears' boyfriend David Lucado.