Well the drug dealers were certainly there first, etymonline.com has this to say of "user": Of narcotics, from 1935; of computers, from 1967."

I am struggling to find a source for who first called users "users" back in 1967 - Actually I see it being used in the 1965 "Multics" papers [0] and the 1964 article in The Atlantic "Computers of Tomorrow" [1], as if it was already a term-of-art.

[0] https://multicians.org/history.html

[1] https://www.sas.upenn.edu/~traister/greenbf.html