We use Google calendar, which we set as private, but then share with the other folks in the group that are collaborating with, so they can see from a web view our when/wheres.
Sync's to my DroidX "auto-magically"
Also makes it very easy to set an appointment, and immediately invite someone from your contact list.
If you went with a paid version of Google Apps, there are more features/collaboration to have.
As Wendell would say, did I mention that it's FREE?
