Look up "Network Storage Drive" or NAS
PCWorld sell them and price has dropped enormously in recent years, less than £100. Maplin and even Aldi have sold them before now.
I use a Freecom drive, it has RJ45 LAN and USB connectors and can also act as an FTP server on the internet with passwords and user defined areas (if your router has an internet connection).
Using the RJ45 connector it is just a matter of plugging it into the back of the router (after connecting it to an individual PC to set it up).
There are a few software bugs in mine but it is quite a few years old, I guess by now they are a lot better.
Alternatively, servers for a small business network don't have to be that high performance, a budget PC will normally give more than adequate performance, just a matter of getting a big drive or two in there. Its only when you have loads of users trying to access at exactly the same time that you need high performance. No doubt some people may disagree on this.
I would not recommend using wireless networking unless you really can't do it wired, whilst they can perform well, they also can have quite poor performance for numerous reasons which makes hard work for users and administrators alike.
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