Joining other types of groups may or may not indicate a person’s self-identification. Being a Republican or a Democrat, for example, may just be a matter of whichever party your parents belong to. Spending time with a particular group of friends may only mean that you like only one of the people in the group or you have nothing better to do, it may have nothing to do with your self-identification. People that belong to these groups do not necessarily belong to any one social group for identification purposes; it may be only a matter of convenience.
It depends on the individual and the particular social group under discussion as to whether we primarily identify ourselves through association with social groups. Once the individual or the social group has been identified, a determination must still be made as to whether the individual has joined for self-identification purposes or to impress upon others a certain image of that individual.