Are there any free solutions that would allow you to have a private, collaborative feed. Say I'm trying to plan an event with 4 friends and each of us has certain jobs, something that would allow us to "update" each other on what we've been doing (e.g., "Jack just booked a bar," or "Jane just got a band.")

I'm not sure there are any tailor-made for this but you could use something more in the task management family like do.com or asana.com to assign and manage various things that need to be planned/organized.

do.com
asana.com

Those both look promising, thanks. Any other thoughts, Branchers of the world...