If a user tries to open a Workbook that is already open, then they normally get a message saying that the File is locked by a certain user and would you like to open the file in Read only mode. However, you can open a workbook in shared mode by following these steps. First open the file that you want to share, then once the workbook is open, go to the Tools menu and choose Share Workbook from the drop down menu. The Share Workbook dialog box will now appear. To enable sharing, you simply click once on the Allow Sharing check box, so that it has a tick. Once you do this other people will now be able to use your Excel workbook whilst you are working on it.
You will find that there are dozens of different tutorials available online, ranging from books and courses that you can pay for to tutorials and video lessons that can be downloaded for free. Each of these courses is broken down into sections that will address each of the different sections of formula so that you can learn them in groups. This tends to make it much easier to fully grasp the necessary formula for each section before moving on to the next segment.