in the very bottom left corner there is a little arrow icon like this:
<<
click on that, and it will display your tracks (seperate) and below that, your grouped tracks. click on your "guitars" track, for instance, and it will temporarily un-group your tracks so you can mix them further without affecting the group itself. click again to re-enable your group.
or, just right click on the group and remove the group, and re-do the group if all else fails, do it again.
hope that helps