When a new user is added, it temporarily gives them a password of "booking" (as set in Admin/Other Settings/Global Settings).

The first time they login, they will be automatically directed to their Edit My Details page where they will need to set their own password before being able to proceed.

If they ever forgot their password, there's an option on the login page where they can reset it themselves. Alternatively, this can be done by an admin in User Manager.