One thing that make a coop very fun is the possibility of make a quest with friends. If you put a "Share Quest" option it would make it very simple to find partners. Like... "Everyone in your party that can do the quest will receive a message to accept the quest." That makes it easy, your partners wont need to go to the npc. Sorry about my english, not native language.
Here! http://pastebin.com/e0TGyN0P first "group" of answers Tiy: the complete story, quests, universe, will all be available in multiplayer