I would like to create a network backup drive for my 4 homeusers, have network with a wireless router and an external hdd. Anyone ? who has done this before ?
If your router is capable of acting as a file server (it would have to have a USB/esata port, but it also needs support from the router MFG to enable the router to act as a file server), that would be the only way to do what you want to do without spending any money. If your router has a USB port, but doesn't enable fileserver features, you can try open source firmwares such as Tomato (there are others, but that is one I've heard thrown around alot), which MAY support it.
If your router doesn't natively support it, you can buy an external hard disk enclosure that has an ethernet interface or another external hard drive with an ethernet interface.
If you go for the enclosure route, you can take apart your external hard drive and pull out the actual hard drive, and put it into the enclosure (this will save you the cost of the hard drive). Or you can just buy a network accessible external drive, and use your current external drive for something else.
Be aware that network accessible storage like these carry some security risks: if your network is not secure, or is breached, anything stored on the networked storage is easily accessible. I personally leave mine unplugged unless someone wants to use it, for security and simply to save power. They are however very convenient for sharing things between computers, I use mine fairly often.
Edit: I missed the obvious alternative free solution. You can attach the drive to the computer that you leave on the most often, and share it across the network. This has the same security risks. You also are required to have that computer on to access the device from other computers, and it's a little harder to setup (especially if you have computers with different OS's on them). If you want to use this method, look up "Windows file sharing".