Wunderkit requires a registrations. Once logged in users can then start creating their own workspace and name it before inviting others via Facebook and Twitter to join it.Any workspace created can either be open to public or just private so that only you and your members can see it.
Wunderkit has also a social part where users can also add comments, notes, like status updates and start discussions. Inside a workspace there are three built-in apps and each one of them will help you organize a different aspect of your project in a truly simple way. No need to install anything they are ready for use.
Head over to Wunderkit and give it a try.