I've updated the Wiki on Github based on this suggestion!
Hm. Are you on Windows or Mac? When you type the command to start Jamulus Server with the "-r" flag in the command prompt or terminal, do you see any information in the console as the server launches? It should confirm your flag selections. If not, maybe there's a typo. On the Github wiki, there's a page with the configuration options. Looks like it's supposed to be a capitol 'R' not lower case 'r'. Could that make a difference? https://github.com/corrados/jamulus/wiki/Command-Line-Options
Jamulus does the recording. Jamulus creates a WAV file for each audio source (each musician who was present in the session). Jamulus also creates one .RPP file (which is a Reaper session file) that organizes all of the WAV files correctly on a timeline so they are properly aligned with each other. You can use the WAV files without Reaper, but if you do, you'll have to align them with each other using a different Digital Audio Workspace such us Audacity, Garageband, or Pro Tools. Also, I believe that...
I haven't tried out 3.5.5 yet. I was responding to @wysocki's question. I interpreted that they were asking about the general recording feature, which many Jamulus users don't know about.
When you run your own private Jamulus server, you can specify an option to enable a session recording by adding the -R flag on the command line and including a directory to save the session in. For instance, on my Windows machine, when I want to launch the server in recording mode, I open up the Command prompt and run C:\Program Files (x86)\Jamulus\Jamulus.exe -s -r "C:\Users\Nathaniel\Music\Jamulus Sessions\".