You probably want to start using Bus tracks. You create and label a new Bus track, then assign the outputs of your drum tracks to direct to the Bus. You still have control over the levels, automation, and effects of the original audio tracks feeding into the bus, but have additional control to mute, solo, or adjust the overall level of the Bus. You may also add effects to a bus, which can be convenient if you wish to apply the same effect and parameters to several different tracks - the incoming signal is mixed, then passed through the effects chain in the Bus, before continuing to the master output.
My understanding of a mix group and what my current manuals on Audition tell me about groups in Audition seem to be rather vastly different.
That would probably be the 'manual' then - the process is essentially identical!
The only thing that Durin didn't mention was how you actually assign the channels. The easiest way, once you've set up the bus, is to go to the track output routing at the bottom of each mixer channel (where it normally says 'Master' and assign that particular channel to the bus there.