To bus two or more instrument channel strips in a patch together for group volume control you can create a new (stereo) Aux in the mixer panel, set its input to the next unused bus, and set the outputs of the instrument channels to that bus. (If there's a reverb or other send&return effect included in the patch and you want to control its level along with the level of the instruments, don't forget to route its output to the same Aux as well). If you also want to have a screen control (like a volume pedal) control that Aux level, (create one and) select it in the Workspace and assign it (in the Screen Control Inspector at the bottom) to that Aux > Volume. You could then assign an external controller like a MIDI volume/expression pedal (or a fader or even the modwheel) to that screen control, for live performance. (see the screenshot at bottom)
Of course there are other ways to do it, like assigning the Volume faders of the multiple instrument channel strips to the same screen control (by adding additional mappings in the Screen Control Inspector), but the Aux approach seems to be closest to what you seem to be looking for.
I don't know what your level of expertise is, but if these aspects of MainStage are unfamiliar, the video below covers the basics of screen control assignments & mapping..