In Beta Readers, click Send manuscript (or the paper-plane icon on a row).
Enter the reader's name and email, pick the book, add a draft label such as "Draft 2", and set a due date. Upload the manuscript. Accepted types are .docx, .epub, .pdf with a text layer, .html, .txt, and .md, up to 25 MB. Choose where the reading starts and ends so front and back matter are skipped.
The reader gets an email with a private link.