At any point, you can add users to your plan. You will need to add users to your plan in 2 cases:
In order to add user to your plan, first open the main menu and select Preferences.
In the User section, select "Invite users".
Enter one or more emails you want add to your plan, then select the level of permissions you want to give those users:
Optionally, you can select one or more folders on your plans you want to give those users access as soon as they accept your invitation.