I don't know that there is an actual CWRPS primer out there. For how huge it is, this fandom is very decentralized.

Jensen's friends, who are primarily musicians and sometime actors, often pop up in stories alongside other people from other WB/CW shows, primarily One Tree Hill (due to Chad and Jared being friends and Danneel and Jensen dating). It's a pretty tangled web. Feel free to ask for further clarification and I'll provide any info I can.

Despite that, it's a lovely huge fandom with lots of great fic. Enjoy!