Lounge access is one of the priciest extras in travel — American Airlines now charges $750 a year just for a solo Admirals Club membership. But there's a loophole of sorts that gets regular American flyers through the door for a fraction of that: becoming an authorized user on someone's Citi / AAdvantage Executive credit card.

Here's how the maths works. The card charges a flat $175 a year to add up to three authorized users. Split three ways, that's $58.33 per person — less than a tenth of the solo membership fee. Even if you're the only authorized user eating the full $175, you're still well ahead of $750, and there's a bonus the cheap membership doesn't offer: guests.

Authorized users get their own independent Admirals Club access, so they can use lounges without the primary cardholder anywhere in sight. Entry requires the authorized-user card, a government ID and a same-day boarding pass for a flight operated or marketed by American, Alaska or a Oneworld carrier. Better still, each authorized user can bring either immediate family (spouse or domestic partner plus kids under 18) or up to two guests at no extra cost. The $750 single membership, by contrast, covers the member only — no guests, no partner lounges.

There are limits worth knowing before you sign up. Authorized users don't get access to American's partner lounges, which rules out select Alaska Lounges in the US (think San Francisco and Seattle-Tacoma) and Qantas Clubs in Australia. They also miss the perks reserved for primary cardholders: the Global Entry or TSA PreCheck fee credit worth up to $120, priority boarding and check-in, a free first checked bag on domestic American itineraries. One upside — spending on an authorized-user card earns AAdvantage miles and Loyalty Points, all credited to the primary cardholder's account, helping them chase elite status.

For primary cardholders, adding users is straightforward: online, via the Citi app, or by phone. Each authorized user receives their own card in the post, which doubles as their lounge ticket.

Who should actually do this? Frequent American flyers who mainly want lounge access and can piggyback on a friend's or relative's account are the clear winners — TPG's editor-in-chief says he uses it to give American-flying friends and colleagues their own entry. If you can't find a primary cardholder to attach yourself to, compare the alternatives carefully: the Citi / AAdvantage Executive itself carries a $695 annual fee but includes a full standard Admirals Club membership (guests and partner lounges included) plus all the travel perks, and new cardholders can currently earn 125,000 bonus miles after spending $15,000 in the first five months. That's still cheaper than the $1,200–$1,400 standard membership on its own.

The catch, as ever, is that the cheapest route requires a generous primary cardholder and fits only travellers loyal to American and its Oneworld partners. If your routes and lounges line up, under $60 a year for lounge access with guest privileges is hard to beat.