C. F. Keil and Franz Delitzsch on Numbers 11:26

Num 11:26

But in order to prove to the whole congregation that the Spirit of the Lord was working there, the Spirit came not only upon the elders assembled round Moses, and in front of the tabernacle, but also upon two of the persons who had been chosen, viz., Eldad and Medad, who had remained behind in the camp, for some reason that is not reported, so that they also prophesied. “Them that were written,” conscripti, for “called,” because the calling of the elders generally took place in writing, from which we may see how thoroughly the Israelites had acquired the art of writing in Egypt.