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-customize-category).gnus-category-kill).gnus-category-copy).gnus-category-add).gnus-category-edit-predicate).gnus-category-edit-groups).gnus-category-edit-score).gnus-category-list).