A group of Mormons from the Mormon Battallion were working at Sutter's Mill to earn money to take back to Utah with them. They found the gold and helped set up the initial extraction operation.
There was an extreme labor shortage at the time, but I don't know that Sutter wouldn't have eventually been able to get some help once the gold was discovered. Gold tends to attract labor.
Once the gold is found, there's going to be a Gold Rush with or without the Mormons. Finding a labor force won't be a problem (although getting them there will be initially until SLC and other waystations are established along the westward route). For a really cheap, disposable labor force in early California, there's always the Chinese.