With the Uno phone app on the home page, you can tap on the bottom right message icon with the word social under. Tap the search tab on the left side. Tap the second tab that says Find and enter your friends' ID numbers OR share yours so they can add you.
If you're looking for a fun card game to play with friends, give Uno a try! Each player gets dealt a hand of 7 cards to start. To play, you'll match one of your cards with the card that's been dealt from the draw pile. The first player to get rid of all of their cards wins the round.