I think this happens based on the script writers where they develop segments to involve and engage the crew members. They are obviously aware of this ahead of taping.
I am not sure it constitutes as them becoming an actor and being part of a union such as SAG or any others.
For example, Andy (The producer of Ellen's show) is credited as the executive ...
To Join SAG, you must have done some previous work so showing random crew members on air should be allowed espically if it is a one off appearance.
From SAG website on what is needed to join:
Here's what you need to qualify:
Proof of Employment
SAG-AFTRA membership is available to those who work in a position covered by a SAG-AFTRA (or AFTRA or SAG) ...
visa, check, authorization, official permission
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Il modulo domanda di revisione viene compilato dalla società di produzione o distribuzione del film e presentato alla Direzione Generale per il Cinema. Successivamente la Commissione di Censura visiona il film ed emette, con apposito Decreto ...