Play music in the group you want to modify, click on the group and click on the ‘fader’ icon in the upper right. Then, in addition to changing the group volume, you can also separately increase or decrease the volume of all the speakers in this group.
NOTE: Only possible when music is being played in the respective group.
You also can change the the volume of a single speakers by tapping the speaker in the start screen of the Google Home-app.