Org friends are a collection of people who’ve been added to your Blissbook account (usually because they’ve been granted access to something), who are not members of your organization.
The main use case for the org friend list is if you have multiple people with access to multiple documents, and you want to revoke their access from everything all at once. You can archive that person on your org friends page, and it will revoke their access from everything. This way you don't have to go into the audience for each individual document.
Adding someone as an org friend doesn't do anything in itself. It only does something if you assign the person you're adding to a group that has access to something. For example, if you have a handbook or document shared with your EMEA group, and you add someone as an org friend and place them in the EMEA group, then that person will be gaining access to that handbook or document. In this example (and in all instances of adding an Org Friend directly), you are adding that person to your account manually so no automated email notification would be triggered. If the person you are adding is gaining access to anything, Blissbook will prompt you to ask whether or not you'd like to send a notification.