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 and press 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.