was told that those helos have a crew of four. So, perhaps two Recon or Seals on board?
More likely additional squadron personnel.
There is a pretty demanding air operations schedule that Marine helo squadrons must fly in order for their helicopter pilots to stay certified on all the various aspects of military helicopter operations: night operations with NVGs, over water navigation, deck landings on amphibious ships (day/night (illuminated and NVG), etc.
In discussing this with Marine aviators (helicopter type, one each) during and after my time in the Marines, it became clear that a lot of the helo flights made during long transits to and from deployment locations was driven by the need to keep up pilot qualifications. Out of that necessity, the birds were going to be flown whether or not they had passengers in the back.
The two extra passengers might have been squadron pilots rotating with the pilot and co-pilot to log night flying hours.
CH-53s don’t crash well. Hoping for the best, but it rarely happens with this aircraft type.
Would Recon and SEALs be on a training mission like this?