Unlike banks, credit unions are membership-based organisations. To borrow or save with a credit union, you have to become a member. Their services are not open to the general public as a whole.
Anyone can become a member of a credit union, as long as they fit within that credit union's 'common bond'. The common bond is a list of characteristics that every member of that particular credit union must have in common—such as having the same employer, living in the same community, or working in a particular occupation.
Each credit union's common bond is defined in law and forms part of their rules. So, unfortunately, if you do not fit a particular credit union's common bond, they'll be unable to allow you to join as a member.