Probably Agaricus subrutilescens
or Agaricus augustus

Joe posted on Mushroom Observer

Notes: Agarricus but what species? I was going to guess A. subrutilescens,
but then noticed that cap flesh stains yellow near cuticle when cut.
Both Arora and Trudell & Ammirati say that A. subrutilescens,is non-staining. In duff (and possibly bark dust). Nearest tree was a spruce.
I do not have a mature specimen, so cannot tell real maximum sizes and other characters.

Dick said: Have you ruled out A. augustus or A. subrufescens?--

Joe said
Dick said: Sometimes we just have to accept that we can't tell for sure between closely related species. The microscope can often help us out but not always. I quite often say it's probably "this" because the variability factor is always in play. Especially with Agaricus.
I would agree that A. subrufescens seems unlikely but the cap color does match pretty well. I would also agree that A. augustus seems the most likely. Your photo of the lone cap does seem to indicate a little purple color but the group shot does not. I would definitely expect to see some purplish tones if it were A. subrutilescens. Sometimes you do not see the scales until the cap has expanded.
Contact Judy Roger for some KOH. I'm going to get some from her in the next couple of days and could pick some up for you also

John said: One idea to think about is that Arora describes subrufuscens as common in S. California, rare elsewhere