Whenever a non-agent raises a ticket in your helpdesk, they get added as a requester. If you have the user activation email for requesters enabled, they can set up a password by clicking on the activation link. However, if you don't have it enabled (and do not want to enable it), but you still want a few of your requesters to be able to access your helpdesk, you can change/setup passwords yourself.
If you are an admin, account admin or any other agent with "super admin" controls, you will be able to change a requester's password by going to their page. Requesters will not receive any automatic notification about the password change.
However, agents will not be able to set passwords for a requester who doesn't have an email address associated with it.