How to effectively create common documents (Word, Excel, Powerpoint...) collaboratively?

With common file sharing systems like SugarSync or Dropbox it is not effective for a group to work on the same document at the same time because one cannot know what part another one is working on at the very same time. Furthermore, parts of the document can be lost: 1. Person A opens the document and starts working 2. Person B saves the document with changes made after person A opened it 3. Person A saves the now obsolete document. So the part of person B gets lost. How to address the issues mentioned above, how can multiple persons work effectively and collaborative on the same document?
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