if you want to play on this server and you downloaded this server's client and started at the cave, then you should know that some locations are such that you can't be in there with another player unless you explicitly bring them to that location given that it's possible.
This way the game protects you from trolls and pks when they're not helpful. Once you get out of the cave and get into the lander's camp, you will be able to see the other players there.