Well, pretty sure you have to meet each other somewhere at first. Meet yourself at the portalseekers gateways. From there you can basically go anywhere together.
You can meet eachother by warping.
There are multiple Worlds (aka servers) based on different regions.
I have absolutely no idea why this is not a major information found in the game, but with some google-ing.
Click me for Information about regions
The best way is mostlikely building 2 warp conduits and porting to each other.
I have no friends playing Boundless, so I can’t screenshot/record you hot to and for some other reason no one yet made a video about it, but in this one is a little part about it. It’s not really explaining it but quickly shows you how to warp to your friends