The official university calendar has a definition for 'member' but in this context we talk of members as anyone (staff, student or visitor) who has an ongoing relationship with the university over a period of time. A significant amount of work has been done to define who is (and who is not) entitled to membership of the university when based upon the criteria that define the purpose for which you may be here and the job function that you may have been asked to fulfill.
The types of membership were identified by the original Membership Project which ran from 2000 until 2004. The categories that it identified are still in use today. Most types of membership are provided with a university card which identifies you to the university's systems across campus.
Membership is not discriminatory other than upon the specific criteria that have been laid down and agreed. Membership is not unilateral. There are many types and grades of membership which allow access to different university resources and facilities. The pages from the membership project explain in more detail which types are available and the selection criteria.