Recon platoon securing the LZ and waiting for the rest of the battalion to arrive. The smoke grenade is used to mark the LZ. The RTO or Platoon Leader would broadcast "Smoke out!" and for security the incoming aircraft would identify the color of smoke. In this case, they would radio back "Identify purple" and wait for confirmation. This was done because there were times when the VC were monitoring American radio frequencies and popped smoke to try to decoy or confuse American troops. The platoon leader, 1st Lt. Jim Deegan, with his map in front of him, is talking to his RTO, who was never more than a step away in the field.
The burned out grasses are due to the artillery prep before the combat assault. This whole series of photos were taken in early '68 on a mission where I took my camera along, which was unusual. Since on a typical combat assault each man carried anywhere from 50 to 75 lb. of equipment, you tended to leave anything you didn't actually need.