I have a blackjack game(console game with visual c++), and I was wondering, how do I put in multiplayer?

well it would need to be over a network, unless you are relying on peoples sense of not looking at their friends card so they can own them.
With networking, you need to send messages to the clients telling them what the other players have done, so it doesn't all go out.

so can you give me a quick example?

well no... cos I don't really know network programming.
XD

Basically:

1. Deal out cards - send data of cards to all clients, so each clients' game knows who has what cards.

2. Start with the first player - their actions need to be sent to the client, so that they all have the same game going!

3. Repeat step 2 for all clients, until turns are up.

4. Check the value of all the cards. The host can send out the data of who wins to the clients, or the clients can check for themselves, though it's easier if the host does this so things don't go out of sync.

5. Restart - Reset everything and go from the top.