I would like this as well. Maybe even just a Group setting where one could add the chat channels to which members of a group would be auto-joined, similar to how many of the multi-select settings work.
Similar to this but I think different enough. The one below is more about auto-creation of a chat channel for the specific group:
https://meta.discourse.org/t/chats-for-discourse-groups/223967?u=jimkleiber