Plannr is an online, email/mobile based plan-maker and event coordinator.
When you invite someone to a plan via email, Plannr sends them one email on your behalf. The email contains all the details of the plan– the time, location, and who else is coming. When you update a plan, the change is pushed to your friends via whatever contact settings they have set. This can include email, txting, or a push notification to the iPhone app.