In the manuscript editor, select the words you want to talk about, or put the cursor in a paragraph, and press Comment in the toolbar (or Ctrl+Alt+M). Write what you have to say and save it. The words are marked in the text, and the Comments panel beside the chapter lists its comments in the order of the text.
Answer, resolve, reopen. Everyone who works on the book can reply to a comment. When it's dealt with, resolve it: it leaves the text's marks and waits under Resolved, where it can be reopened. Comments are resolved by whoever wrote them, and by the owner, the authors and the editors.
Comments hold on to their words, not to a page. A comment keeps the words it's on, with a little text before and after them, and finds them again in the text as it reads now: after edits around them, in a new version, or when the chapter is split or moved. If its words are changed or cut, it's listed under Text changed with the words it was on, so nothing said is lost.
Beta readers. Under Beta readers in the editor, add your readers by name and choose how many days their links work (60 unless you change it). Each reader gets a private link of their own, shown once: copy it and send it yourself; the studio never writes to your readers, and keeps only a scrambled form of each link, so a lost link can't be shown again. Make a new link for that reader instead: the old one stops working.
A reader's link opens the manuscript as its current version reads, chapter by chapter: your working copy stays yours until you save a version. Unless you chose reading only, they select words and leave comments, signed with the name you gave them. Each reader sees only their own comments. Theirs come into the editor's Comments panel, marked as a beta reader's, to answer among yourselves and resolve like any other.
The list shows when each reader first and last opened their link and how many comments they left. Stop a link to end it, or remove the reader, which takes their name away; what they said about the book stays, unnamed. While your plan doesn't include beta readers, their links pause (each reader is told), and they work again, with every comment kept, when it does. Whenever their link is open, a reader can remove their own comments, even once you've made the link reading only.
Who can do what. The owner, authors, editors, designers and reviewers comment and reply; a client reads. You edit and remove your own comments, and the team's owner can remove any. Comments stay with the edition; they never go into the ebook, the print files or the source package.