Once you head into the party, wait for Lester to send you a picture of the Croupier. Check the picture in your phone's messages and then look for an identical person in the party. The Croupier will likely be on the ground extremely drunk. So try to look for passed out people on the ground. As you get closer to the Croupier, he/she will be identified (you will see an arrow on top of them) and you will get the option to search. After that, if the card is not on them, you need to search for it in the surronding area. Initially it will not be marked with an arrow so just look for a highlighted/glowing object. Once you get closer to it, you will identify it and just grab it.