As an Admin, you will have the option to set a password for your customers, using which they can log in to your Support Portal. 

To do this, please go to Customers tab > click on a contact name > Change Password. On the following page, you can set the new password that the customer can use for future logins.

If you are using Freshdesk on Mint, you will find your contacts under the Contacts tab from the left panel.