Once you collect enough duplicate Pins you can upgrade them by selecting them in your Pin collection and choosing upgrade. This will cost a few coins and give you experience in return. The quickest way I found to get this was to either buy the common bundle in the store. I was able to get 50 duplicate Pins for 200 coins.
The other option is running some of the starter Hordes a few times. Some of them drop 25x Grenades if you complete them. If you do this with a few people in your crew you will have it in no time. Even if someone else uses the item to start the Horde you still get the Pin reward.