From a structural perspective an individual actor is examined according to their structural characteristics (gender, age, ethnicity, class) and is assumed to behave in accordance with these structural characteristics.
The subjectivity of the actor is insignificant. Ethnomethodology on the other hand, attempts to highlight the subjectivity of the individual actor and thus needs to identify the person in a way that acknowledges their knowledge, competence, engagement, commitment, or ability to make sense.
The term member accomplishes this. Ethnomethodology also refers to membership categories (things like teacher, mother, employee) and identifies membership categorization devices and rules of application (things like the economy rule and the consistency rule) as a form of ethnomethodological analysis.
Competency and participation in acquiring a mastery of language: a reconsideration of the idea of membership - Michael A. Forrester and David Reason
Abstract: For ethnomethodology and conversation analysis the concept of 'member' or 'participant' remains central. The aim of this paper is to consider a number of ideas originally outlined by Garfinkel and Sacks (1970), and by way of extension and clarification, discuss transcript extracts from recorded everyday interactions between two parents and their pre-school child. We note that membership, or what might constitute being a member, involves possessing a mastery of language and being able to produce and recognise glossing practices. Furthermore, a member is someone who recognises that the actions which make conversations possible are reflexively accountable practices. By looking at extracts where a child is 'learning how to talk' we find evidence in support of the suggestions that: (a) membership is indeed a dynamic and concerted accomplishment in context; (b) adults often treat children as 'good-enough' members; and (c) infants can attain membership status not only with reference to displaying a mastery of language, but possibly by displaying a mastery of communication. We close by noting that one of Garfinkel and Sacks' (1970) particular insights was that in displaying mastery of language, speakers display membership, but mastery of language is itself a concerted accomplishment in occasion precisely because speakers display membership by not drawing attention to the fact that they are indeed a member.