Adding an FTP user is great if you want to give out access to a specific folder on your server, without allowing access to the full server. It isn't however recommended to create multiple FTP users all accessing the same folder (such as your main site root) as this can cause lots of permissions issues. It is recommended in this instance just to use one single FTP account - you can always reset the FTP password each time you've finished accessing it.
To create a new/multiple FTP users, please follow this guide.
1. Login to your Plesk control panel.
2. Click on Subscriptions. Click on the subscription/domain you'd like to add an FTP user to.
3 - Click on the FTP Access button.
4 - Click on Add an FTP Account.
5 - Enter the details of your new FTP user. Make sure you use a nice and secure password - random letters, digits and symbols - this is very important as it is part of your site security. You can use the Gnerate button to automatically generate a complex password, use the Show button to make it visible. Select the folder you'd like the FTP user to have access to. You can create a folder via FTP using the master account first if required. Click 'OK' to save the user.