Is there group camping and rates for non-profit groups like the boy scouts? We are coming up after school on a Friday, at least a 1.5 hr drive away. What's the latest check-in time? For late arrivals, can rangers or park attendants let us in?

You need to contact the city of Glendora for a permit to camp at this location. The gates are locked at sunset, you need a permit.

Camp Williams is a mobile home park. They have a campground area but it's been unkept for a while

There is no check-in time and there are no non-profit group rates. You need to go down to City Hall a permit and you also need to buy insurance for the site. If you're looking for an actual campground to camp in I would check out camp Williams in Azusa Canyon. Or call Glendora City Hall