Technically, the license says that you may install up to five copies on computers that you own, it does not say that you can share the program with four other people. It may be the TurboTax is using a new copy protection scheme that prevents more than one instance of a license number to be used at any one time. Were you trying to install the second copy while the first copy was running? You may need to shut down the first computer to install the second copy, or at least disconnect the first computer from the Internet if you want to install the second copy while the first copy is running.
Another issue that crops up is when people buy pirated copies from non-authorized vendors who reuse license numbers.
