Yes Carlos is right, it depends on what type of document this is. If it is MS Word the document can be modified to allow multiple users access simutaneously. Other applications may not allow this functionality. In that case the one with modify rights will be able to change the document, others will need to save their changes to a new document and re-modify the existing original when it is available to them. In the case of a document which allows multiple users to modify, each will be prompted to save changes made by other users when they try to save.