Subgroups utilise a bus send to assign multiple channels to one subgroup channel control, usually a stereo channel with independent left and right faders. This allows us to control an assigned amount of instrument channels with one hand and with the multitude of things and engineer must take in to consideration with a mixing console, whilst listening to and watching the band, this may make things more manageable (especially in large band scenarios).
To engage subgroups, you must locate the in-out switches that connect the sends to the group channels, usually located just above the faders near mute, PFL and any other sends on the mixer. Once switched in it is usual practice to disengage the main mix sends to ensure that the is a single control for the mix. From this point, the audio is being sent directly to the group channel controls and these controls can be adjusted.