First, the user needs to create an account at the site. Click the login link in the top right corner of the page, then click the register link. Make a note of the user name used when creating the account.
Using an FTP client, connect to your website. Navigate through the site folder structure to the BackOffice directory in the site root. Download or edit the web.config file. In the "users" attribute of the <allow> element in the <authorization> section, add the username of the user to whom you want to grant access. Note that the value of the "users" attribute is a comma-delimited list, so you will need to add a comma after any other users in the list before adding your user.