Direct Messages is more fully featured with new controls for adding and removing people. If someone doesn’t have to be in the conversation, you can easily remove them. Removal cuts down on their social networking noise and yours. Also, when you navigate from a direct message notification, you’re taken right to the related message. These changes are available in Lightning communities and in Lightning Bolt solutions.
A new user permission, Add People to Direct Messages, is available to enable a user to add others to direct messages the user is in.