Use 'group' var

That somewhat cryptic label means that if this button is on,
moving a point will result in all points with the same glyph and
color being moved along with it.

The 'group' variable is the indicator variable that identifies
the group membership of each point according to the current
brushing states; it's defined on the brush panel.

However, you don't need to turn brushing on in order to define or
adjust the 'group' variable; that will be done for you here when
you select this button.