We had 11 in our group and possibly one of the best tour leaders there is! (a nod to you Seth!) I came alone and was with a mixed group of single travelers and people who came together. We all got on really well, it helps that we were all pretty chilled out and relaxed - we did things as a group or we were happy to do our own thing alone too.
This is a busy little trip - expect to be spending a few hours a day in the van (bonding and sleeping time), late nights and very early mornings to make the most of everywhere you go. My fave bits were hiking in Yosemite, helicopter in the Grand Canyon and shooting in Las Vegas. There is so much to see in a short space of time, but it is the best holiday I've had so far. You won't need much money in the National Parks, but can rinse through it in the Cities! We had a virtually free night out at Mandalay Bay Light Club thanks to the included party bus and Seth getting us onto the guest list with free drinks.... awesome night!
Have a plan for what you want to do on your free day in Vegas so you don't end up wasting it - I went shooting and toured the strip. Go with an open mind, relax, take it all in and you will definitely get along with everyone. We spent our first night sleeping under the stars (too hot to get in the tents!) next to the Colorado river at Lake Havasu.
Only downsides were - you do not see much of the beginning and end cities at all so be sure to arrive a day or two early and leave a day or two after the Trek (some good hostels at Venice Beach and in San Fran.) I regret hardly seeing anything of San Fran as I left the day after the Trek ended.
Overall, definitely glad I decided to book thi trip. I will be booking another Trek for next year, heading back to see more National Parks and getting a real look at San Francisco.
So worth it and I get wanderlust every time I look at the photos I took!