Select your group of shapes and instead of making them a group go to the Object menu > Path > Add (Cmd + option + U). This will create a 'Union' instead of a group and can be used as a Mask.
Credit goes to Reinoud Schuijers below, I'm just reiterating as it seems people are still having problems.
Select your group of shapes and instead of making them a group go to the Object menu > Path > Add (Cmd + option + U). This will create a 'Union' instead of a group and can be used as a Mask.
Credit goes to Reinoud Schuijers below, I'm just reiterating as it seems people are still having problems.