It depends on your hardware and software setup, as well as the platform, as well as (to some extent) what you mean by sharing.
The simplest way is with NFS (networked filesystem). You export the filesystem or directory you want to be shared. this is the system that actually has the disks with the data attached to it. You can specify which systems the directory is available to and whether it is read only or read and write.
On the receiving systems, you import the directory/filesystem you have exported. You specify a mount point on the local system and this is where users on the importing system access the disk.
Check the man pages for nfs (man -k nfs to get a list of topics) to see how your platform implements it. The commands vary by platform, but the basic concepts are the same.
Another way to share a hard disk is with systems that boot off a hard drive that is somewhere on the network. I don't know how that is done. Maybe someone else can help.
And another way to "share" a hard drive is with hardware that allows two systems to be hooked up to the same disks. Generally, you don't want both systems accessing the disks at the same time because of logical volume manager considerations.