Hi there :)
The easiest way to do this might be with an affiliate app from the Shopify App Store. Here are a few examples:
- Omnistar Affiliate Software
- ReferralCandy
- Refersion: Affiliate Tracking
- LeadDyno: Easy Affiliate Marketing (I know this one lets you put a "sharing widget" on any product page)
There may be manual ways of doing it as well, as this forum thread discusses.
Hope this helps!