Then your friends can join you, with drop-in/drop-out co-op, so as Joe Staten was playing through there in the E3 stage demo, he had one friend join, and then another friend join, and they had three people playing still just in that private area. Then they went into an area of that world that is a public area, and those are areas of the experience in Destiny, where as your Fireteam moves towards those areas, the game is finding other Fireteams that are in that public area, and then matchmaking people together, and putting them in the same game.
But you’re still on your mission, and you’re still able to just ignore whatever’s going on there, and just keep going on your way; and sometimes you’ll have objectives in these public areas, where you’ll have to complete some mission objective there. Or sometimes you might just be travelling through them, and you’re like “Oh, there’s a sweet public event going on; I’m going to stop and go check that out”, and maybe you’ll meet some new friends, and hang out with them for a while, or whatever else, but then you can go back to your mission if you want to.