To challenge a friend, navigate to the friends area via the friends button at the bottom of the screen. Here you will see your friends that are online and can send them a challenge request.
 

Alternatively, if the friend you would like to challenge has not yet downloaded the Grand Gin Rummy 2 app, you can send them an invite by tapping on the green invite friends button. 
 
Next, tap on the green invite button and you can forward the link to your friend by copying and pasting or via an external application (email, text message, Facebook, WhatsApp, etc.).
 
As soon as your friend accepts the invitation, you will receive a notification and your friend will automatically be added to your friends and can be challenged.