I don't know the groups, but they could be heading to the river trail. Not a lot of choices out there unless you want to head to Chino Hills. Not the best riding as the pastures do not smell good, the roads are fair at best and I can't remember any bike lanes.
Mando...what river trail are you referring to? I'm a fairly new transplant to that general area and had never heard of a river trail out there.
Well, the Santa Ana river trail starts at Waterman/Hospitality Drive in San Bernardino and ends at Tyler Street in Riverside. It then starts back up at Green River next to the golf course and goes down to Huntington Beach. 30 miles each way. They are in the process of connecting the two halves now.
To start at Green River, you take the 91 West to GR. Turn right and park along the street by the Jack in the Box or there is parking further down the hill by the golf course. You'll see where once you get there. Ride your bike to the bottom of the hill and get on the trail.
You can ride there from Prado, but it is a bit more complicated. Like I said before, not a ton of good riding in Chino or Chino Hills so the SART may be a good place to try.