How can I configure Nightbot for my viewers who just arrive on my stream?

Nightbot doesn’t have the ability welcome new followers or people who have just entered the channel. This is often seen as spammy for the chat and it is not something that will be supported.

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Considering that this isn’t possible it likely wasn’t what you think. You can make a nightbot command that automatically responds to messages like “hello” but its not possible to do exactly what you described.