A user has exited my campaign, is there any way to re-add them?

Once a user has exited a campaign, there's no way to re-add them. You would need to duplicate the campaign to re-add them.To stop users exiting your campaign, I would recommend including a "dummy message" in your campaign, so that they do not meet the "received all messages" exit rule.A dummy message is a message that cannot be sent due to its audience rules - like Email is unknown, for example.