For getting access to the EDITOR role (CMS from the client side) they do not need to have a Webflow account even. You are creating Editor on the site settings panel, add his e-mail and send the invitation. Then the client who got that invitation will get the link for creating his user/password to the Editing site.
Client will see the only site (not a Webflow dashboard or the Designer mode) but will be able to change any element, that has checked an option "Collaborators can edit that"
or any dynamic element from the collections.
Hope I was able to explain.