Sometimes the recruitment is direct with clear messages, but more often it is discrete and covert. Such recruitment can start with an invitation to a closed chat group that appears in an online game community, or with a link invitation to a closed forum. From there the messages slowly become more explicit propaganda and recruitment.
It´s also a common strategy to post funny memes and images in online forums with subtle messages that have an influence on participants´ views over time. The more extreme examples are groups or individual sympathisers that explicitly propagate committing violence.
It is important that practitioners become aware and understand the importance of online work in preventing and countering violent radicalism. Check out some good practices and successful approaches in a recent article on delivering interventions online on VOXPol.