I've been removing extreme amounts of machines from my base, nearly everything, but I was only able to remove 4Mb from my 18Mb save file. My friends can join my game if I create a new save, but I still can't join theirs if they have a new or old save. Hopefilly they bring out a patch in the following week so I could continue with multiplayer, even though I've basically finished all of the content the game has to offer so far.
Also, I don't think it's time depentant as I have tried at so many difderent times throughout the day and night to play with friends.
Cheers.