When you create or join a custom created game, you will find a button "Invite Player ..." at the bottom of the dialog prior to the start of a game:
Clicking on the button will bring up a list of users you may invite to your game, including users on your contact list and the game lobby. If there are AI computer players available, you can invite them to your game as well. Some AI players will decline joining your game if the settings you chose for the game are not to their liking.
Tip of the day: Invite and play with AI computer players in your created custom game