You may have noticed that sometimes you can't find your friends even though you're on the same server. This is likely due to the fact that you're in a different channel than them.
In order to switch channels, please go to the World Map by pressing the "View" button and press down on the "Left Analog" stick. This will bring up a menu that will allow you to change channels.
Once you're in the same channel (and server) as your friend, you should be able to party up and play together.