Client only need to authenticate when canceling/rescheduling. It is optional when booking new appointments (unless you restrict your services to current clients only).
In any case, a new password can be obtained from any login form (by clicking the forgotten pwd link and providing the associated email address) for people who don't remember what it was or did not set it themselves and did not receive the original email.
Note: the password is case sensitive and 0, O, o are all different. So are i, I, l, L and 1.