How to change the format of the lines in Group buffer?
You've got to tweak the value of the variable gnus-group-line-format. See the manual node "Group Line Specification" for information on how to do this. An example for this (guess from whose .gnus :-)):
(setq gnus-group-line-format "%P%M%S[%5t]%5y : %(%g%)\n")