Microsoft today announced that real-time co-authoring is now available in Office Web Apps. So, now you can contribute and edit documents simultaneously with your friends and family in the Word Web App, PowerPoint Web App or Excel Web App. You can now see the real-time presence which helps you see where your co-authors are working in the document. Also, there is a new ability to see changes to text and formatting as they happen which will help you and your co-authors stay on the same page.
For example, your whole team can choose to work in the Web Apps for a highly collaborative working meeting to get a shared project done. But what if somebody on your team isn’t available? That teammate can choose to make revisions when he’s able, even from the Office applications on his computer. His changes will sync up to the document stored in the cloud when he saves.
And if that teammate goes offline? He can continue to work in the document using the full power of the Office applications and when he gets back online, his changes will be synced back up for the team to see. So now, when working together with Office you get the collaborative power of the Web with the feature depth of the desktop applications on your PC, or Mac, for a no compromise collaboration experience.
Source: Office blog