For a long time the way to get free United Club access has been to match status to Turkish Airlines Miles&Smiles. They’ve been inconsistent over the years in how they’ve accepted matches (first via email and then via web form), how long they’ve taken to reply (quickly vs months), and what status they’ve been willing to match (sometimes United status, sometimes only competitor alliances, sometimes only top tier status). But it’s status that lasts two years.
This is least useful with Skyteam which has both Alitalia and Aeroflot currently offering matches. Skyteam status doesn’t get you Delta lounge access on domestic US itineraries.
If you’re a Star Alliance Gold member, though, 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.
What’s interesting is that Star Alliance member Copa, which used to use United MileagePlus as a frequent flyer program but recently launched its own, is status matching both United elites and elite members of other alliances. Per Miles to Memories:
Send an email to firstname.lastname@example.org and request a match. Status matches are not listed under the reasons to email this address, however hopefully someone will forward the request off. Call: Contact the Copa Service Center at 1-844-287-0304 between 7am and 11pm CST Monday-Saturday to ask for a match via the telephone.
It makes sense why Copa would offer status to United elites, since Copa’s frequent flyers are United MileagePlus elites. I wouldn’t be surprised if they started to institute a rule, pressured by United, that a minimum number of Copa segments be demonstrated before becoming eligible for a match. But so far there appear to be successful reports from United elites as well as others.