Edit: I'm happy to share and am available for further questions, but for anyone planning to visit the any parts of the Southern California desert during the summer, please please don't plan to go all out on your first visit. Please take a measured approach. For example: an overnight car-camping trip before you attempt a backpacking trip. When you're new to the summer-desert, plan on carrying one-gallon-per-person of water for a hike of any length, even if it's less than a mile; it can save your life if you unexpectedly get heat exhaustion when it's over 115F.
Edit II: To volunteer, e-mail AB.Sheepcount AT parks.ca.gov -- express your interest in the 2020 census and they'll put you on the mailing list. If you'd like more information, reply to this comment and we'll work out how to chat directly.
To be honest, I'm not sure how to find them. I somewhat fell into this one, but the term to search is "citizen science". https://www.citizenscience.gov/catalog has a catalog of Federal events. The one I do is more at the state level. There are also non-profits that often work closely with regional state and national parks.
