Nielsen Homescan opens for new applicants every few months, it seems like they are looking more often than that though. They are now accepting applications again and all you have to do is answer a few questions here to see if you qualify.
If you’re selected, they’ll send you a scanner and you scan the grocery items UPC codes and that transmits the info to them. You then earn points and can get some great rewards. I have been on the waiting list for about 3 years now so they must have plenty of So Cal panelist for now. You should still give it a try, they may be accepting panelist in your area.