The way it appears to me is that the campaign missions are really only like a tutorial for levelling up anyway.
Perhaps its just me but it's not as if the story is especially interesting or gripping. I haven't really followed any of it.
Judging by reading articles about the game, most people race through all the stuff outside the Dark Zone just to complete it and then spend all their time inside the Dark Zone where the encounters are more interesting due to the interaction with other players.
I don't think it should matter if the group contained people with different levels - as long as we're not talking about a level 20 with level 10s.
If it's like a level 14 with a level 10 then the game should still be able to provide a balanced experience.
And, for the record, I don't have a second character and I'm still (level 10?) the same level as when I last logged in. lol