DCA groups on the M Series consoles can be thought of the same as VCA groups on an analog mixer. By placing channels in the same DCA group you can have quick access to their overall level to the master bus with a single fader. For example once you have set gains and levels for all individual drum mics you can set all to the same DCA group for control over the level of the entire kit on 1 fader. 

To assign channels to a DCA group first press the DCA button in the layers section:

Now the right side scribble strips will be populated with DCA Groups 1-8. Press and hold the select button of the DCA Group you wish to use then press select on the channels you wish to assign:

Every channel you press will now be assigned to the DCA and you control the level of all those channels to the main output with a single fader. The DCA mute button will also affect all the channels. 

If you are setting this up in the application then you can either use the small matrix above the channel strip to assign DCA or use the Channel view at the top of the screen.