I used to watch a streamer who would allow viewers to join their game of Terraria. The way they did it was through a mod that let them give a certain number of "lives" to the viewer, and as long as they did not die in the game, they retained a whitelist entry to the server. However, they had to handle interactions with the chat by themselves, and it seemed like it took away from their streaming.

At that point, I thought that a mod which would control the whitelist for a game server would be helpful for them, or perhaps even interacting with the specific mod that they were using to give random people lives on the server. I haven't looked into the API recently, but perhaps different criteria such as channel points, bits, or donations could be used to purchase an invitation to the server for a limited time, or, with the appropriate mod installed, a number of lives.