The way NMS tends to work with Multiplayer is friends can just drop in and out of your sessions, you do not need to start a dedicated MP session.
If people cannot join your game, in my experience it could be a few things.
-Your current status on Steam (are you set to invisible or offline for instance)
-Your Multiplayer settings within the menu, you choose who the game is open to, or even if you want MP on, check your settings in game.
-Could be steam account privacy settings, I would need to sanity check this with someone who is more familiar with the nuances of our networking.
What I can say is that if your friends can join you when you host a game the way you are, they should just as easily be able to join you games otherwise, unless settings your end are preventing them.
Many thanks