Dear student,
It seems that you are getting confused with regard to the term group activity as used in the features of management.
Management is said to be a group activity because various individuals in an organisation must work together towards the common goals and objectives of the organisation. In this the management plays an important role in the sense that it is only through proper and effective management that the individual efforts in an organisation are synchronised towards the common goals.
Whether there are one or more managers in the organisation it is their responsibility to unify and synchronise the individual efforts towards the common goals of the organisation.