Usually, they are people who have made arrangments, ahead of time, to visit a specific cast member backstage. When I had the opportunity to visit backstage at the Ambassador (Chicago) this past Dec. 2016, I was told by one of the cast members to go to the stage door and announce myself on the intercom button that "I'm a guest of ______". I went down this long pathway (almost an alley, really), took a left and went inside where the stage door person confirmed I was a guest of. Soon afterwards, I met one of the Billy Flynn understudies. I noticed that there was very little space in the area behind the stage. He then showed me where to go and we proceeded to the actual stage. I felt like Eve Harrington (without the hidden malice). I had been on a stage before (Xanadu) and had been to a star's dressing room (Patti LuPone in Sweeney), but this was the first time I had an extended tour of an actual theatre stage. That was the happiest post show experience I ever had. He asked one of his cast member friends to take some pics. Dylis Croman was there as well. I took a jazz hands picture with her. That was a trip. I also asked for AFW's autograph, which she graciously provided. Essentially, you have to be acquainted with one of the cast members of a show, in order to gain entry past the stage door and onto the backstage. If you ever get the chance, it will be a magical experience.
"Noel [Coward] and I were in Paris once. Adjoining rooms, of course. One night, I felt mischievous, so I knocked on Noel's door, and he asked, 'Who is it?' I lowered my voice and said 'Hotel detective. Have you got a gentleman in your room?' He answered, 'Just a minute, I'll ask him.'" (Beatrice Lillie)