Hi Anders,
You currently can only add 1 user at a time.
Users can create items from the front end, once they have logged in. The k2 login module has "create new item" link in it, or you could create a custom "special" menu item per user, that only they would see, that would give them access to their User Page and that create new item link.
Hope that helps.
Simon
K2Joom.com