After an account has been created, you will receive an email with access details.
You can start the sharing process with uploading a document/file.
In this guide I am going to share a PDF file by clicking on the '<' like icon next to the file.
Then I choose testUser2 (the current username will be emailed to you) and 'notify by email'
Having a username to share the content with makes it unnecessary to tick the 'Share link' checkbox.
(This checkbox would be used if you wanted to share with somebody who does not have account in the system)