You'd normally do all category maintenance from the category buffer.
When you enter it for the first time (with the J c command from
the group buffer), you'll only see the default
category.
The following commands are available in this buffer:
gnus-category-exit
).
gnus-category-kill
).
gnus-category-copy
).
gnus-category-add
).
gnus-category-edit-predicate
).
gnus-category-edit-groups
).
gnus-category-edit-score
).
gnus-category-list
).
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