As mobile networks expand rapidly to facilitate the rising number of mobile phone population, more mobile social services are being developed and offered. To create an efficient social functionality, characteristics of mobile social network must be studied. Social closeness is one of the basic fundamentals of any kind of social networking. In this paper, closeness in mobile social network is the subject of the study, from which social grouping scheme is proposed and validated against the feedbacks of human subjects. Based on the proposed grouping scheme, a study of the impact of the mobile social closeness to the similarity in calling patterns and reciprocity is presented.