Hiya, you can click xmit to turn off transmit, that way you can play along, when you have something you feel goes you can then re click xmit and join the jam.
It's not good to sit for long periods not transmitting though, people will become annoyed if you sit silent and never play.
But most will unxmit until they have something that fits then xmit to join the jam.
Some servers has Radio Streams and
http://ninbot.com has a lounge for each server so you can chat and listen or play along to get used to things.
Welcome to NINJAM.