When attendees are asked for a password to register, Eventsforce strongly recommends they use the "Request login details" link to get a "password reset link" emailed to them as this is most secure. As an admin, you may need to manually set a password for a person in the system.
Setting the password as an admin
As a last resort, you can set the password for the attendee by taking the below actions:
Go to Reports > Quick Search
Enter the email address for the attendee
Select the "Search all events" check box
Click "Search"
Click on the "i" icon to open the personal details page
Click the "Reset password..." button
Set the new password then click "Submit"
Other articles you may find useful: