Groups federate much more effectively than hashtags, so they are a useful alternative to hashtag following. Group posts reach all the group's followers no matter which server they are on. This makes them very useful on very new servers that haven't built up many connections yet.
I haven't listed any example groups by @-ing as I don't want to spam any group's members. However, if you want to post to a group all you need to do is @ it.
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