In all messaging, allow the coach to pin important messages.
In all messaging, include a search bar so previous messages can be found easier for both the coach & client rather than needing to scroll through all messages to find a specific one.
In both direct & group messaging, allow old or no longer relevant files to be removed from "files" by the coach.
In group messaging, add a tab for "links" so the coach can provide clients important links can be uploaded & removed when no longer relevant & clients can easily find them.