You're either headed for network-attached storage served from a Mac or served from a Network Attached Storage (NAS) server — either of which will be limited by the speed of your wired network — or toward what's known as a Fibre Channel Storage Area Network (FC SAN) configuration with the Apple Xsan software — which is faster, but more complex.
Any Mac can serve storage to the network via the local System Preferences > Sharing settings, though configurations running OS X Server have user interfaces and tools that make this sort of file-serving task easier.
For some more details here, see the OS X Server product overview.
If you use FC SAN, you'll need storage that connects to the FC SAN, as well — a FC SAN is akin to a network connection and shared storage, but the SAN is dedicated to storage access, and most any recent FC SAN configuration will have host bus adapters and storage controllers that are vastly faster than a Gigabit Ethernet network.
Storage on FC SAN can either be served to your Mac systems via — for instance — Thunderbolt to FC SAN connections from each client, or can be connected to some subset of server hosts and then served from those hosts out to the local network via wired Ethernet or (less desirably) via Wi-Fi; as network-attached storage.
Thunderbolt doesn't permit having multiple hosts, so you can daisy-chain up to six boxes from one host, but — because OS X and Thunderbolt support is not a connection intended for multiple hosts on one Thunderbolt bus — you can't connect additional hosts to the bus. There's no Thunderbolt splitter. You can serve storage from Thunderbolt via the network by configuring file shares on the Mac that's connected to the Thunderbolt bus, and mounting the disks from the client systems.
Apple does indicate "servers made easy" or such in their OS X Server advertising, but you'll want to have somebody familiar with Ethernet networking, IP configuration and addressing, DNS configuration and management, and other tasks available — OS X Server is easier, but configurations in this area can still be or can become complex and comparatively arcane. SAN configurations add complexity, too.
You might want to have a chat with the local Apple Business folks, if you have an Apple Store around.
FWIW, the forum that this question was originally posted in is for Xserve server hardware and RAID storage hardware, and no Xserve server supports Thunderbolt. Xserve and Xserve RAID were connected via FC SAN (using Xsan), but they're rather archaic hardware at present and not really something most folks would probably be interested in a new deployment.