Here is the post of the Portal Seekers. In the very first entry you have a map which shows which planets are there and which is connected to which. … ** UPDATED ** Guild Portal Seekers / Hunts / Contracts/ Gateways / Recruiment … each has at least portal or a hub (collection of portals together) through which you can go to the planets.
If your friends are already in the game ask for the world and go there. They should also give you their coordinates so that you can walk there. But don’t forget to save the location of the portal in the locations menue cuz being able to fast travel to it with a warp then makes traveling and trading way easier later 
… .And if traveling through the portals try to avoid the upper ones on the map (tier 4+), because those have hostile atmospheres and you slowly die by suffocation if you stay longer then your oxigen bar there.
If you are friends in the game allready you can select an option to start near a friend, but I dopn’t know if they have to be online to use this function.