Hi mattyj45,
Multiple people in one location can use jammr together through just one computer. That's easy to do if you already use a mixer to combine instruments/microphones when practicing with your band.
When I have a friend over we play on jammr using this approach. The guitar and a bass can be connected to the computer through my multi-effects pedal (it has 2 instrument inputs and acts as a mixer).
Do not connect multiple computers in the same room, make sure to send everything through one computer only. You would hear different time-shifted versions of the jam if you use multiple computers in the same room. You can read more about how jammr is “live” but not “real-time” on the Frequently Asked Questions page. See “How does jammr work?” and “Why is jammr not real-time?”:
https://jammr.net/faq.htmlOn a related note, if you're new to jammr you may not know that jammr works great for improvization and practicing grooves but is not suitable for rehearsing full songs. This is because it's an interval-based jamming system - you need to stick to a chord progression and cannot change from verse to chorus (for example) without causing a collision. If you were planning to play full songs then you may need to look elsewhere.
Stefan