Sign in to app.speechify.com using your Speechify account credentials, preferably using a Chromium browser.

The home page is your library. Click on the three-dot menu under the file that you want to share.

Click on 'Share' and this will generate a link for the file.
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Click on 'Copy' to copy the link and share it with another Speechify user.
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Please note that this link can be opened in the Speechify app only.