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Introducing <a href=http://subeta.net/forums/groups/>Forum Groups</a> to the forums, where you can create a group and allow people to join.
Each group has a hashtag (#) which when called on the forum, pings everyone. For example, if your group has the hashtag and someone posts that on the forums, it'll call everyone in that group.
The focus of the groups is to make it easy for you to get called to a thread that might interest you.
Seconding what Fai said - it would be really good if the groups people are in didn't show up automatically on their forum posts. I'm already seeing a few people who are in about 20-odd groups, and it can stretch out the white space at the end of their posts something silly. Great new feature, though.