The very first set of instructors were the original test platoon. These 48 men formed the cadre for the 501st battalion but I can assume they were taught by those who had come up with the techniques. The initial training cadres were formed by the early experimental battalions. Later training cadre were men selected from an older unit to form the nucleus of the new unit. Usually around 10% of the new unit would have been formed from the training cadre. In the case of the airborne units these men were already jump qualified or knew how to jump qualify people.

The 506th seems to be an odd case since they weren't assigned to a division yet. Some of the cadre stayed with the unit but 99% of them remained behind to train the 501st. They had more than enough men coming through the ranks that they could be replaced withou much effort. Sobel was just a normal infantry officer. He wasn't airborne qualified and thus had to take the course with the rest of the men. Those from the cadre who were already jump qualified took an extra jump master course while the other were in the prejump training weeks.

I'm going to make a second post since I'm on mobile and it doesn't seem to want to save my later edits. Anyway I edited the intro to include that some men from the 501st PIB volunteered to form the cadre for the 502nd PIB in July '41. At the end of January '42 these two units were upgraded to full regiments and there numbers were filled out with new volunteers but don't appear to have been jump qualified right away. Men from these unit were pulled to form the cadres for the 504th-507th PIRs for their activations between May and July but it seems that most of these men stayed to rejoin their respective regiments as they passed through training. By January '43 all of the major PIRs and GIRs were at least designated for creation.