I wish I could assign a task to users that are not using ToDo and it sends them an email to manage the task(s) via a webpage. I think this could be a marketing technique for you!
Hopefully, I still have your ear. My thought process...
First off, I don't think that security is the primary focus for a feature like this. The purpose would be to add productivity to the ToDo Cloud user. That being said, security should always be part of the thought process.
1. Create a "Guest" registration system using the person's email address as a key. If a task is assigned to said Email... everyone is in good shape an a secure task has been assigned and can be completed.
2. If you try to assign a task to an unregistered Email... RED WARNING appears and warns you before assigning the task it is not secure. When person is sent an email with the task it would contain a single button to close the task to keep it simple. It could also refer to any other tasks associated with that email address. If more tasks are associated with this email address, the guest should be able to register and claim the tasks. Didn't start secure, but are now.
I really do think this could have an added benefit to your customers and pull in additional customers. That couldn't hurt.
This would be an amazing enhancement. Easy for folks like me that work with people only willing to use their email to manage their day. (grrrrr)
Today, I assign tags for each person that may get the assignment because you can't assign a task to someone not using the product. I will Email them the task, but of course I have to wait for the Email back saying it is complete. The ideal situation is for them to simply click a single URL to mark it complete.
The marketing comment is dead on. Certainly, many folks will start to use the product if their boss was assigning them tasks.