The default operation of WP-Members™ has a two stage registration system that requires users to enter a valid email address when registering. Upon successful registration, a randomly generated password is sent to the user via email which they can then use to login.
If you have established as User Profile Area, the user can go to this section to reset their random password to whatever password they like.
Alternatively, you may add a password field via the field management panel if you would like users to pick their own password at the time of registration. To use the password and confirm password fields, enable them for display in the plugin’s Fields tab.