See also the official documentation:
Help Center > Members Manual > Working with chats

Chat Overview

Who can create a chat depends on your group settings (see Admin->Settings->Features->Chats).

Any group subscriber can "join" an ongoing chat in that group. A "private" chat, with only a subset of the group, is not an available feature. It is also not currently possible to make a chat viewable by non-subscribers.

The chat window does NOT display chat activity in real time in some browsers. To see new content, you may have to refresh the page periodically. 

You cannot post a photo to a chat. The only images that appear are subscriber's profile avatars.

With the exception of chat creation, ongoing chat activity does not generate any email notifications. One must log into groups.io to initiate one, post to it, and return to monitor it online periodically. 

Chats can later be closed or deleted by an owner or a moderator with the right permissions. Closed chats remain visible but you can't add to them. They can be reopened, if desired. Once deleted, they are gone.

See also https://groups.io/g/GroupManagersForum/topic/23482824#10266