If your account is linked to Facebook, you'll be able to invite your friends by going to Menu>Friends and searching in the search bar. You'll see the "Invite" button next to their name.
You'll also be able to do this by going to Menu>Facebook>Invite friends.
You won't be able to invite friends that don't use the app if your account is not linked to Facebook. If you wish, you can tell them to download Trivia Crack and, once they have an account, follow them so that you can play and chat with them. In order to do this go to Menu>Friends, write their user name in the search bar and tap on "Follow".