Group messaging will be introduced sooner or later (ref.).
In all likelihood, you will be able to add new users to a group chat after it has been created. If they can read what has been said before, that could be either useful, because then they won't miss out on important stuff, but it may also contain sensitive information (also: temporarily removing someone to discuss something without them would then not be possible).
Should new users be able to read messages that have been sent before they were added?