Questions about multiplayer progression.

I’m going to buy the game and I want to play it in co-op with my wife, but I’d also like to keep exploring solo. How does multiplayer progression work? Can I create one save to play solo and invite her to join that same save? Or do I need two separate saves running in parallel, one online and one offline?

Another question: can I use Steam Family Sharing to play with her while buying only one copy of the game?