The ANT accepted him as an apprentice.
The deal was not generous. In exchange for membership in the company, Poitier agreed to serve as the theater’s janitor — cleaning the space, maintaining the building, doing the invisible maintenance work that kept the institution running. He was an actor, sort of, and also the guy who mopped the floors.
What the ANT gave him was exposure to serious theatrical craft. He watched rehearsals. He studied how other actors moved in space, how they listened to other performers, how they inhabited scenes. The ANT had trained Ossie Davis and Ruby Dee. Harry Belafonte was also an apprentice during the same period, and the two of them — Belafonte and Poitier — would form a friendship and mutual influence that would run through both of their careers for decades.
The story of how Poitier first got onto a Broadway stage involves Belafonte directly, though not in a way either of them planned.
A rehearsal. A Broadway casting director or producer present in the theater to observe. Belafonte, for whatever reason — illness, another commitment, a miscommunication — absent from the rehearsal. Poitier, who was there, who knew the material, who had been watching from the side, stepped in.
The casting director saw him. He was cast.
The specific production is noted in various sources as Lysistrata, an all-Black Broadway production in 1946. Some sources also mention Anna Lucasta in 1948. What matters for the arc of the story is not the specific title but the pattern: a young man who had been turned away from the same institution less than two years earlier was now performing on a Broadway stage.