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 and registration > Emailed login links > Allow registration via registration page

Whether registration is on or not, all users who have been invited into the workspace can order a login link to their email. Uninvited users can register themselves into the workspace as participants only if registration is enabled.