gnus-group-prepare-function

gnus-group-prepare-function is a variable defined in `gnus-group.el'.
Its value is

gnus-group-prepare-flat

• This variable may be risky if used as a file-local variable.

Documentation:
*A function that is called to generate the group buffer.
The function is called with three arguments: The first is a number;
all group with a level less or equal to that number should be listed,
if the second is non-nil, empty groups should also be displayed. If
the third is non-nil, it is a number. No groups with a level lower
than this number should be displayed.

The only current function implemented is `gnus-group-prepare-flat'.

You can customize this variable.