The FSPN is currently a totally open and decentralized network and no steps need be taken to gain access. You simply have to download an appropriate p2p client application and connect to one of the central servers listed on the main page. If you are able to serve content you can acquire the content from other nodes or traditional means. However, if you serve content, please serve both the content and a valid openPGP signature on the content from the content's developer or primary distributor.
Since FSPN is a not a physical network, you can participate in a less direct manner by connecting to any p2p network (gnutella, freenet, etc) and sharing freesoftware and openPGP signatures.
The only differences between a central server and a regular node is that a central server is listed on the web page of the FSPN, is available 24/7 and provides either some free software on the server or provides pointers to other servers with such content.
Clients use central servers to hook into the peer to peer network. Right now, central servers in the FSPN can use any software they wish to serve their content and may carry any content they wish.