Why and how to join a group

Some categories are closed and can only be used by their specific groups. Other categories can be used by any forum user. For those, a group is not strictly necessary. But it might still be useful to keep an overview of the people interested in some area, and also you can message and mention the whole group via @groupname, then every group member will get a message.
The open groups you can join by pressing the “join” button, the closed groups via the “request” button here: