Sharing needs to be re-thought. I would think that it should be the most important way new customers are won so special attention should be paid as a first touch point.

Here is the problem as I see it.

1- You want to share a task with a non-member.
2- You invite them.
3- You have to keep bugging them to find out whether they have joined (or repeatedly try to add them as a contact).
4- They join; as they don't really know the site the come in see nothing and leave.
5- You then try and share a list, but can't because they haven't added you as a contact, even though your the reason they joined.
6- They are expected to figure out how to add you as a contact although the only incentive they currently have is the idea that you want to share a task with them.
7- They add you as a contact (after being pestered for days)
8- You share a task.
9- It shows up in their Inbox and they look at it and go back to using Outlook

Here is how I think it should work:
1- You share a task with a non-member
2- You are prompted to add a message in the invite that accompanies the details of the tasks
3- The person receives an email with a link to the sign up page
4- They enter their details and they are prompted to add the referrer as a contact and the big message box says 'Your Friend Is Sharing X With You >> Click Here'
5- They are taken to the task and a little box below gives them quick tips of what they can do next and sells them on entering their own tasks.
6- A task or note or something notifies the referrer that the person has joined and have been added to their contacts.

I have invited two people and both times the person came and left and didn't take the time to even look at the site.

I know a lot of users come in through the front door, but referrals are always a better way to get customers as there is someone that can support and sell the site. Hope that is some helpful feedback.