If I recall right you have to play with ppl you know. I think bungie did not add a matchmaking option to the main story to avoid other players from speeding through the mission and leaving the new player unable to enjoy and appreciate the story.
If you need someone to run the Campaigns with add me if you'd like, I'm gonna be running the campaigns for my son's account for him over the next few days and I know how boring it can get doing it solo