Managers may update the organization's subdomain by going into "Settings" from the user list at the top right of the page. To edit the URL, just click the pencil "Edit" button on the top right of the "Organization Settings" section, and edit the entry in the "Website" field.
Individual user-specific URLs may be distributed by the manager. To find each user-specific link, click on the user on the "Settings" page, and copy the URL in the top section under the field titled "Website."