“Join this group and invite all your friends,” is a message that children and young people are increasingly seeing on WhatsApp when joining new groups. These can be hubs for illegal content and can be dangerous, says Kira Liebmann, a family coach in Germany advising parents on how they can protect children from threats online. What are these sprawling WhatsApp groups about, how do they work? Kira Liebmann: In WhatsApp groups, many people can exchange text messages, pictures and videos at the same time. These groups can include up to 1,024 members. They are very popular among schoolchildren who…