Final answer:
Attraction is not a criterion used to classify groups in sociology; rather, membership, strength of social ties, number of members, and type of contact are considered. Attraction is more about relationship formation than group classification.
Step-by-step explanation:
The criterion that is not used to classify groups is attraction. When sociologists classify groups, they consider aspects such as membership, the strength of social ties, the number of members, and the type of contact. Although attraction can play a role in the formation of relationships and can determine with whom people become friends or form romantic partnerships, it is not considered a foundational criterion for the classification of groups in a sociological context.
Moreover, as detailed in the discussion about high-risk activism and social ties in the civil rights movement, strong social ties are crucial for deeper engagement and commitment in social movements, contrasting with weak ties such as those often formed through social media platforms.