Usher, Stevie Wonder Sing at Baller Pre-Wedding Bash

8/2/2014 11:10 AM PDT BY TMZ STAFF
Usher and Stevie Wonder performed Friday night at one of the most baller wedding parties of all time ... and the couple getting hitched wasn't even famous.

It went down last night at the St. Regis Monarch Beach Resort in Dana Point during an epic carnival themed rehearsal dinner ... filled with games and a HUGE Ferris wheel.

The couple -- Kaily Smith and Adam Westbrook -- more than likely paid for the event with the father of the bride's checkbook.  He's Mike Smith, a big wig in the natural gas game who's worth a fortune.

To keep things on the beach DL when it got late ... people in the crowd had to wear headphones to hear the music.

We're told the couple rented out the entire hotel from Thursday through Sunday ... and one source told us the whole thing, including the wedding, should cost upwards of $5 mil.