If you want to add your friends to your neighbor list, you can simply add them as your friends either via Facebook or Game Center, and they will automatically become your game neighbour when playing. Make sure that you're logged in when you play Horse Haven: World Adventures, so the friends list is updated.

If you want to add some random players, pay attention to the cars driving by your farms. You can click on the small plus sign and invite those users to be your neighbors! You can also add friends through a club when you have joined one!