In the brush palette, you can go to Auto Masking and set Mask By Polygroups to 100. This will setup an automatic masking so that when you use a brush, it will only affect the polygroup.
To try this, press CTRL and paint a mask on an object.
Press CTRL + W to turn the masked area into a polygroup.
Select the Move brush ( B + M + V )
In the Brush palette, under Auto Masking, set Mask By Polygroups to 100
Now, when you use the move brush, it will only move the polygroup because everything else is auto-masked.