A Group Relationships refer to a polyamorous relationship in which several people form a single unit. All participants are in a relationship with all other participants of the group relationship. This relationship may be exclusive to those in the relationship (not an open relationship) and the people in the group may consider themselves married to each other.
Not all polyamorous relationships are group relationships. Some members of a polyamorous relationship may not consider themselves in a relationship with all other members. (For example A, B, and C are dating. D is dating B and C, but D is not in a relationship with A. In a group relationship all four would be in a relationship with each other.)