All the people who HODL are forming the HoldGang, so a mass of people who refuse to sell their coins ... in most cases, HODLers are holding Bitcoins or the most promising altcoins such as Ethereum, Litecoin etc ...
The driving force behind joining the Holdgang is the fact that bitcoin has a limited supply, so if enough people refuse to sell it, the supply available for sale will go down, which should drive it's price up.
Proud members of the Hodlgang let the world know about it by using the hashtag #hodlgang in their social media posts.