If you’re a Star Alliance Gold member — with an airline other than United — you can use United’s lounges.
While United Gold elites and higher get access to the lounges of non-US Star Alliance partners in the US they do not get United Club access unless they’re flying internationally. But Star Alliance Gold members of all other frequent flyer programs (like Aeroplan, Turkish Airlines Miles&Smiles, Lufthansa’s Miles & More, etc) do.
Star Alliance member Copa was status matching both United elite members and elites with other alliances and giving not just Star Alliance Gold status (and thus United lounge access when flying United) for free but also confirmed upgrade certificates that can be used on United.
Copa has in many reports tightened up a bit, wanting accounts with addresses in their region before considering a match.
However another Star Alliance airline has started offering status matches broadly: Avianca.
They are even offering matches up to their top elite level. Email promo.amigo -at- avianca.com.br with a copy of your elite status card, your full name, and your Avianca Amigo FRIEND number (you’ll need to register for one before submitting). Matched status will be valid through March 31, 2017.