Yes you can, and it it incredible simple. Select the channel within the group that you wish to adjust. A white border will outline the channel to indicate it is selected. LONG PRESS the ADJUST knob to temporarily remove the channel from the group. The white outline of the selected channel will flash while the channel is temporarily out of the goup.  Use the ADJUST knob to adjust the volume of the channel and press the ADJUST knob to return the channel to the group.