at this point, yes, they'd have to be using this too. in the future, though, we were thinking of implementing sharing with unique URLs (like youtube does it with unlisted videos) that don't require login/registration.
should be a core feature, otherwise you'll have a chicken/egg scenario (i do a lot of referrals - but most of my business contacts dont, i would need such a tool, they wouldnt)
if you haven't already filled out a beta invite form, please do. we'd love to chat w you on how you go about handling referrals.