Currently Webflow does not have a single login button for all users.
Typically, it does not make any difference whether a user of an online platform has admin rights or is simply an average user (in the case of Webflow this lower permission level is an "editor".)
Mysteriously, Webflow requires an editor to know, off the top of their head or via a bookmark, the URL to log in and edit the site. They can't click on the login from Webflow.com in order to enter credentials. If they try this, they will be informed that they don't have an account. That needs to be fixed ASAP.