What are the requirements to be able to leave a reference for someone?

I have done work with several guys here on BP. I have also met up with several in person. I would like to be able to leave a reference for these folks as a way of thanking them and encouraging others to reach out to them . How can I go about doing that? I see the spot for it but it doesn't give me an option to leave one. Is there certain criteria they have to meet?