It is not really magic; they just know how to use the groups well. The members in those groups are very active, so when someone shares a post, many people like and comment on it quickly. This first wave of engagement helps the post spread to more users. They also share content that is simple, emotional, funny, or surprising, so people feel like sharing it with friends. Sometimes they post the same thing in several groups to reach even more people. In the end, it is about an active community and content that people really want to react to.