Any Worker who is assigned to a launched Effort on a given date will have access to that Effort. If multiple Efforts have been launched, then the Worker can choose which Effort to start.
Note: Workers must have a data or WiFi connection in order to switch between Efforts.
Here's how Multi Effort Login works on the Organizer mobile app on both iOS (left) and Android(right):
To change Efforts, click on the Menu and then click "Change Effort."