This article explains the difference between login and register.
Login is a default feature that allows workspace access for users that have been invited to the workspace by and admin. Registration is an optional feature that can be enabled to allow anyone (or specified domains) to join/register to the workspace by themselves.
Registration can be enabled from workspace settings > Login, registration and SSO > Users can login with emailed login links > Registration
If registration is enabled for a workspace, the login page will look similar to this:
If registration is disabled, the button looks like this:
Both options allow all users who have been invited into the workspace to order a login link to their email. Only the first option allows uninvited users to register themselves into the workspace as participants.