They need to set up accounts at Upwork and hire you. Upwork will take its % off everything you earn from them.
I'm just curious: what's the advantage to you in bringing them to Upwork? Why not work for them directly and keep that extra 20% for yourself?
You can share your profile link with new potential clients by going to the 'Find Jobs' tab and clicking on 'Profile'. You can copy the link and send it directly to the prospective client. They will then be able to register with Upwork and hire you for a job. Please have a look at this article for more information regarding freelancer fees.