User requesting to be in a usergroup

[turbosatan]

i understand thias may be diggin up an old thread and all but can someone please advise me where this "approval option" is

I know how to reach the actual approval page but how do i set a usergroup to require approval?

i onyl seem to be able to set it to

public - anyone can join doesnt require approval

or

not public - no one can join as its not viewable

i want to have a usergroup which people can request to be part of but that requires approval after some vetting.

plz hlp