You use groups in the same way that you would use fixture selections, for example:
Group
1, Enter : selects all the fixtures in Group 1.
Group
1
@
50, Enter : selects all the fixtures in Group 1 and sets their intensity to 50%.
Group
1
+
Fixture
3
@
50, Enter : you can mix fixture and group selections.
One particularly useful syntax to use with groups is the / key:
Group
1
/
Group
3 : selects only fixtures that are in both groups.