Can I give my signup link to a referral partner, and do they need their own account?

Yes, you can, and no, they do not need an account of their own. Your signup link is tied to your portal, so it does not matter who passes it along: every business that signs up through it routes straight to your library. Your partner simply copies the link and sends it to their own clients, and the client is the one who continues with Google or enters their name and email. If you want to see how much business each partner is sending you, give each one their own tracked link or source channel rather than sharing the same link with everybody.