You have to ask someone if you can have/swap one of their beers.

In theory there shouldn't be a problem with being able to pick and choose, but in reality there's always that guy who shows up with a six pack of Keystone Light and then decides he's going to drink a dozen of the artisan small batch microbrews that other people brought, each of which cost as much as his entire six pack cost him because, hey, beer is beer and if he's drinking he'd rather drink good beer. And since guys like that can't be bothered to show up with good beer if they want to drink good beer, you bring your own beer and get explicit consent before drinking something you didn't bring yourself.