[yscik]

Upon subscribing, you can grant LinkedIn access to Google contacts (via standard OAuth stuff).

This is then used to find your contacts already on LinkedIn, and then followed by a page where you can send out invites for your Google contacts. Of course, if you select everyone and click invite (which is the primary action, but there is a skip button), invites will be sent out.

An "invite your contacts" dialog then occasionally pops up during site usage, where you can once again click "sure, send out invites to all my contacts", but nothing is happening without user consent.

So yeah, growth hacking, but not hacking, like the article suggests.