from what I've read even if you don't buy an expansion a friend can still invite you to an expansion mission/land and you can still play together.
I don't think thats right. This is what I got off a Gamespot interview with the designer
Strain made it clear that they will not punish you if you don't purchase it and your friends do; you'll still be able to play with them, though you won't be able to experience the new content. But they're certainly hoping players will choose to pay in order to continue the story.
See, you won't be able to play the new content, but they'll still be able to play the old.