gnus-auto-subscribed-categories

gnus-auto-subscribed-categories is a variable defined in `gnus-start.el'.
Its value is

(mail post-mail)

Documentation:
*New groups from methods of these categories will be subscribed automatically.
Note that this variable only deals with new groups. It has no
effect whatsoever on old groups. The default is to automatically
subscribe all groups from mail-like backends.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.