Ninjam is open source software to allow people to make real music together
cockos.com
cockos.com
If you'd like to try jamming completely simultaneously, try Jamulus (federated client-server architecture) or Sonobus (p2p). Both are free software and work very well, at least as well as physics allow. In-city jams are hindrance-free, close cities and even close countries are usually painless, as long as everyone's on Ethernet, of course.
The best part about endlesss IMO is that the full 8 channel stems are revision controlled. Every new loop recorded makes a new “riff” revision. It allows rapid experimentation with different musical ideas both solo and with others.
Realearn is an essential extension if you like to use Reaper with control surfaces.
This way you can sync the DAW and any outboard gear to the jamtaba/ninjam clock, and route your pre-existing setup into jamtaba/ninjam.
Personally I'm using Ableton, and it works great
I haven’t tried playing with a bar delay yet. It’s a very interesting idea!
Starting/stopping and big changes in songs can be interesting, but are doable.
Basically you choose the shortest bar length that works well for all players, usually between 8-16 beats.
Jamtaba is a free (and open source) software to connect in ninjam servers
I find its UI is user friendly when getting started with jamming via ninjam, especially when inviting less technical friends to jam.
I made another (somewhat functional!) NINJAM client[1] a few years back as a Chrome App with this motivation in mind. Eventually added a standalone Electron release to work around Chrome's deprecation of Chrome Apps, and sadly ended up abandoning it. I'd love to see a future for it as a PWA if the server software could be updated to handle WebSocket connections (rather than only TCP).