  Pedestrians are often encountered walking in the company of some social
relations, rather than alone. The social groups thus formed, in variable
proportions depending on the context, are not randomly organised but exhibit
distinct features, such as the well-known tendency of 3-member groups to be
arranged in a V-shape. The existence of group structures is thus likely to
impact the collective dynamics of the crowd, possibly in a critical way when
emergency situations are considered. After turning a blind eye to these group
aspects for years, endeavours to model groups in crowd simulation software have
thrived in the past decades. This fairly short review opens on a description of
their empirical characteristics and their impact on the global flow. Then, it
aims to offer a pedagogical discussion of the main strategies to model such
groups, within different types of models, in order to provide guidance for
prospective modellers.
