Telegram and slack bots are probably the best example so far. Bot gets added to a chat and can respond when mentioned in the group chat.
Gotcha, the NLP-enabled version of the good old IRC weatherbot.
For a moment I had a funnier mental image of a chat app with an input field that treats every input as a prompt, and everyone's chatting through the veil of an LLM verbosity filter.
There might be something chat RPG-like there worth trying though ...