PlayStation Share Play: one hour at a time
Worth knowing about, with a real limit. Sony's Share Play lets you hand control of your game to a friend over the internet - "you can both enjoy the game regardless of who owns it" - but a session "lasts for one hour and automatically ends one hour after the visitor joins", and you can use it with only one visitor at a time.
So it is genuinely useful for showing a friend a game, or for taking turns at something single-player, and it is not a substitute for buying a copy of a co-op game you intend to play for months. Restarting a session every hour gets old quickly.
Which route fits your situation
Two of you, different houses, want a proper campaign. A Friend's Pass game. Split Fiction if you are on different platforms, It Takes Two if you are on the same one, A Way Out if you want a shorter cinematic story.
Two to four of you on PC. Remote Play Together. Buy one copy of a local co-op game and play it online - this is the cheapest multiplayer setup that exists on any platform.
A room full of people, or a video call. Jackbox. One purchase, up to ten players, everyone uses a phone.
Everyone in the same room with controllers. Split-screen. Buy the game once, borrow controllers, done.
Everyone in the same room, one controller between you. Pass and play. One purchase, one pad, and the game itself tells you when to hand it over - our pass and play PS5 list has the titles that support it, up to eight people on a single controller.
What none of this covers
Be clear-eyed about the limits. Four-player online games - Helldivers 2, Deep Rock Galactic, Diablo IV, Monster Hunter Wilds, Call of Duty - have no one-copy route at all. Every player buys in. If your group wants that kind of game on a budget, the honest options are to wait for a sale, check what your subscription already includes, or play one of the free crossplay games instead. Our cheap games guide and free hub exist for exactly that situation.
The same applies to most Nintendo titles. Mario Kart, Smash, and Mario Party all cover everyone in the room on one cartridge, but going online means one copy per Switch. There is no Friend's Pass equivalent in Nintendo's catalog.
Subscriptions and family libraries, briefly
Two things adjacent to this worth a sentence each. Xbox Game Pass and PlayStation Plus catalogs frequently include co-op games, so "who has to buy it" sometimes answers itself - check before you spend. And Steam has family library features that let people in a household share games under Valve's rules; that is a household arrangement, not a way to cover a group of friends in five cities, so read the terms rather than assuming. Both deserve more than a sentence, and they get one in our free vs paid breakdown, which covers which platforms charge for online play at all.
The maths, plainly
A premium two-player co-op game bought twice is the most expensive way to play together. The same game bought once with a Friend's Pass is half the price and identical in every other respect. One copy of Jackbox for ten people works out at pennies each. One copy of Overcooked on Steam, played by four over Remote Play Together, is a fraction of four copies on console.
None of that is a loophole. Publishers built these features deliberately, because a game two people can play sells better than a game one person plays alone. Use them - and check the live prices in the table above before you buy, because most of these titles discount regularly.