Ah, this explains it:
"If you would rather selectively choose which types of actions send push notifications from Google+ on your iPhone, you will need to visit your Google+ profile at http://plus.google.com
and choose Google+ Settings by clicking on the gear icon at the top right of the screen. You will then be directed to a page on Google+ where you can choose where and how you will get notified when certain actions are performed on Google+ in relation to you. If you have installed Google+ on your iPhone, you will automatically be able to select settings for several actions, including when someone mentions you in a post or tags you in a photo. You will automatically be notified on your iPhone with a push notification if someone starts a huddle."