How can I set-up a registration and login system for subscribers in my Largo site?

Then you would typically want to add a signup/login link to your site's  global navigation to link to the sign-in/registration page for your site (typically: http://yoursite.com/wp-admin)

Note that simply enabling registration won't necessarily do much unless you're using a plugin to allow users to perform (or not perform) certain actions on your site.

Typically this would be something like restricting access to certain pages only to subscribers, allowing only subscribers to post comments or allowing subscribers to post content (job postings, etc.) directly to the site.