I mean, just like with anyone, people who see a show can tell their friends about it and spread through word of mouth. With certain celebrities, they have a higher profile and might influence people more than your average Joe. Some might even come aboard as a producer to help push the show (Jennifer Hudson, Billy Porter, and Alan Cumming for ASL come to mind)
With regards to paying for seats, who knows? Sometimes they get comps because they have friends in the show - sometimes they might pay. When I saw DANCIN’ for the last time, Todrick Hall was there and his seats were comps (which I knew only because he dropped the stubs in the aisle and I saw them on the floor on my way out)
I know when I met Mrs. Clinton at POTUS, she had a whole security detail with her so I can imagine things like that have to be arranged with the production and theatre in advance.
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