Can only see one of my friend's tracks

[mark@butterill.plus.com]

That's how it works at least at the moment, each user can only see a single track for each session participant. That means each participant must set up a reasonable balance of each track at their audio interface e.g. for guitar & vocals, as their colleagues will only be able to adjust an overall level in their headphones. The session host CAN see all the individual tracks though, by opening the Mixer. That only sets the level for recording or live streaming.
In practice if you're regularly rehearsing with friends in your band the system seems to remember the settings well so once you've each got it set it's easy enough. That includes left and right pan settings - very useful with two guitarists in the band!