An official Google Partners promo video says you must have a Google+ company page to sign up for Google Partners (at 0:47). That’s new.
When the Google Partners program first launched last fall, a company account could be created by someone who could verify she had access to the company’s website. Her options were to use an email with the same domain, to embed a snippet generated by Google which Google could then see and acknowledge her admin over the site, or she could link to a Google+ account that had already done one of those first two. That third Google+ method is even referenced as an option in the same video (at 2:21).