Not sure if reason has busses or way of grouping. At one point of my song I want to control the volume of my drums at another part of the song I want to control the volume of the drums and the synths. Then another time control the volume of the drums synths and bass.

Using more then one Mixer is very useful for making Groups.... Just put the output of, let´s say a 14:2 Mixer for your drums to the input of a 6:2 Mixer...and now you can do a submix of your drums and control the overall volume of the drums by moving the volume knob of the 6:2 mixer.... Same for the pads and the bass..... You also can use combinators with mixers in it for each instrumentgroup and mix them together... Hole that helps...

I now use rewire with reason and bus everything in Logic easier for me.