To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.
Ezuri, Stalker of Spheres
If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
If you proliferate twice, you don't have to choose the same set of players and/or permanents to get additional counters each time.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Fariba Khamseh
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
