I know for a fact if some of your friends do not own the exploration DLC they won't be able to join you in exploration. They will be limited to the tutorial Yland in Exploration until they purchase the DLC. That being said, if they all own the DLC the best way to invite them is in game when you have the map fully loaded. Press esc, then click invite friends. If some of them are having trouble connecting it may be because you are hosting and your computer...

Well yeah you understand collisions and how welds work because you've been playing a while. New players however might get turned off if their computer crashes when they try to make something with 4 million pixel cubes. I know I crashed several times when I first started messing with the game and with editor, but I'm patient like you are. Lots of players don't have patience so I'm guessing that's why the pixel cubes aren't included in the basic shapes...