Dogs are sometimes very vocal when playing with other dogs, and it has nothing to do with aggression. It may sound bad, but it's usually not. Your dog might growl at you when he's being petted, because he's really liking it, or because he's trying to get your attention. These types of growling are "good growling" and are nothing to be worried about. Dogs may also growl, however, when they are asking for space, whether that may be because they are being territorial or because they are scared or uncomfortable--it's important to listen.
For this command, in particular, you want to be sure that we reward the "good growling" and not the aggressive or fearful growling. You don't want to reward your dog for being in an aggressive or fearful state. So, when you are trying to induce the growl, make sure it's coming from a playful place and not a place of fear.
You will need a large space to teach this command, preferably distraction-free. The key aspect of training this command is speed on your part when rewarding. You want to reward at exactly the moment your dog is growling. Growling is usually preceded or followed by a bark, and you don't want to reward a bark, you want to reward the growl. A clicker is a great tool for capturing this kind of behavior because you can catch it exactly the right moment.
You can teach this command to adults and puppies alike and it can take you as little as a day.