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first make your groups in the Group editor ( http://doc.robofont.com/documentation/workspace/group-editor/ )
if your group name starts with a ‘@’ then you can use it in Kern Center ( http://doc.robofont.com/documentation/workspace/metrics/kern-center/ )

Adding a kerning pair hit the plus button at the bottom and the drop down input box should show you all the existing groups that can be used for kerning.

While generating a binary RoboFont ( ufo2fdk ) looks if there is already a “kern” feature present in the feature file. If not it will compile the kerning in side the UFO to a “kern” feature. If you have already declared a “kern” feature it will not be overwritten by the UFO kerning.

hope this makes sense.

good luck