In a multiplayer game, who can see a player's revealed hand?
You reveal the card to all players.

701.13. Reveal
701.13a To reveal a card, show that card to all players for a brief time. If an effect causes a card to be revealed, it remains revealed for as long as necessary to complete the parts of the effect that card is relevant to. If the cost to cast a spell or activate an ability includes revealing a card, the card remains revealed from the time the spell or ability is announced until the time it leaves the stack.