I am interesting in creating a P2P application using Cirrus (http://labs.adobe.com/technologies/cirrus/rtmfpgroups.html).
The examples given create a connection with servers managed by Adobe.
I'd like to do this on my Node.js server, how should I implement this? Or does Adobe force developers that use Cirrus to buy Flash Media Servers?
Well, you could use this open source alternative https://github.com/OpenRTMFP/Cumulus. Another option would be to buy the Flash Media Enterprise Server which is 45,000$ USD. The problem with the opensource solution is that adobe holds some key patents for RTMFP and they have been known to enforce them against opensource such as rtmfpDump (http://rtmpdump.mplayerhq.hu/) - a open source flash movie "downloader", so if your app gets popular, adobe might take actions.
Yet a third option is to start a flash server amazon instance. They are quite expensive, but there are no initial fees http://www.adobe.com/products/flashmediaserver/amazonwebservices/
Edit: It appears they have lowered the price of aws instances with flash server considerably since I last checked, so you might want to look into that, or the open source option if you guestimate your risks of receiving a DMCA take down notice from adobe to be low.