An admin invites people by email from Settings, choosing their role at the same time. The invitee gets a link, signs in or creates an account, and joins.
Invitations use a seat
An open invitation counts towards your plan's seat limit from the moment it is sent. That way every invitation you were allowed to send can be accepted. Revoke invitations that will not be used to free the seat.
Changing and removing
Admins can change a member's role or remove them. A removed member loses access at once, including to their calendar feed. Nothing they created is deleted.
Treat the link as private
An invitation link shows the email address it was sent to. Forward it only to the person it is for.