When accusing someone of the thought crime of the moment, there are two categories. People in one category are clearly guilty, by their own admissions or actions they are proven to be card carrying members of the group under fire. The second category may be admirers and supporters of the group under fire, but they are not members and may not be committed to all of the group's goals. Such people are referred to as fellow travellers.
Saige, for example, admits to being a card carrying member of the ACLU. If this fell into disfavor, then someone who spoke in support of Saige and her viewpoints might be accused, in this manner: "Civil libertarians like Saige and her fellow travellers (such as yourself) would release dangerous convicts into our communities.". Note: Just an example, do not be alarmed.
During the Red Scare this epithet was widely applied to anyone who spoke in defense of the suspected communists being grilled by HUAC and others.