I think I can weigh in on this one!
In the mixer, click-drag, or shift-click on all the tracks in the group. Then go to the output slot in one of the channel strips, and select the next available bus. Since you have all the tracks in the group selected, it will change all the outputs to the bus at the same time.
Once you have changed the output to the bus, Logic will create an aux channel for you, with your selected bus as the input. It's a good idea to rename the aux channel strip, i.e. if it's the drums, change it from "Aux 5" or Aux whatever, to "Drums submix." Then you can hide the members of the group, and still be able to control the volume, panning, etc. of the group as a whole.