Start a message with @silent and it is sent without a push, an email or a sound for anybody; it still counts as unread, and it wears a bell-off mark. In a group conversation, Add friends can make an invite link that expires and can be revoked; its page names only who sent it and how many are in the group.
@silent messages and invite links for group conversations
In Kemble Circle, a message that starts with @silent notifies nobody, and a group conversation can be joined by a link.