gnus-subscribe-newsgroup-method

gnus-subscribe-newsgroup-method is a variable defined in `gnus-start.el'.
Its value is

gnus-subscribe-zombies

Documentation:
*Function(s) called with a group name when new group is detected.
A few pre-made functions are supplied: `gnus-subscribe-randomly'
inserts new groups at the beginning of the list of groups;
`gnus-subscribe-alphabetically' inserts new groups in strict
alphabetic order; `gnus-subscribe-hierarchically' inserts new groups
in hierarchical newsgroup order; `gnus-subscribe-interactively' asks
for your decision; `gnus-subscribe-killed' kills all new groups;
`gnus-subscribe-zombies' will make all new groups into zombies;
`gnus-subscribe-topics' will enter groups into the topics that
claim them.

You can customize this variable.