You should be able to share it the same way you'd share any net connection, you need to link at least one of the Amigas via a network cable to the PC, and then set the PC as a proxy connection for the Amiga.
You'll also need to make sure you've enabled connection sharing on the PC, but that's not always required (it's a PC so anything could happen).
That should give you a networked Amiga/PC, and the rest of the Amigas you can join either via serial or parallel connections - personally I'd go for the parallel, their harder to make/set up but you can get better transfer speeds with lower overhead.