I was recently in my world trying to do all the achievements when a steam "friend" randomly joined. I had no idea that this person joined and that they hid in my game, getting all the achievements I was working for. I think devs should add an alert when someone joins your game since this situation could have turned out much worse. Devs should also add a feature where players can be kicked from a game. I feel like this is an essential part of Astroneer and hope to see it being implemented in the next update.
I realize that these aren't the best achievements, but it's the concept of a steam "friend" joining without your knowledge is concerning.