At the moment we don't have different user categories in terms of listeners and active musicians. However nobody will force you to jam in a session! ;-)
If you're new to sofasession and want to see how it works just join a public session and listen to the other musicians!
Also don't be shy just connect your instrument and start jamming with them, at sofasession you don't have to be a pro! (worst comes to worst they can still mute you, but most will be happy to jam with you)