Assigning tasks to any other person, even without email is an absolute need.
In real life many tasks are delegated to people you talk to, call, and you don't have their email.
Right now you can only assign to doit.im users.
+1
I totally agree, you most certainly cannot limit your life (work and friends etc) to only those who use Doit (or whose email address you know).
But what you can do for now is use a Tag for individual people that you deal with a lot, or for classes of people, such as Suppier.
Unfortunately, though, and for inexplicable reasons, the tags are not visible in the task list, which limits the use of them, but at least you can filter by them, which is also very important.
