Hi Jonas,

It's a bit hidden in documentation - but you need to register them at startup. Inside an initialization module's Initialize method, add the following code to register them. 

            var associationDefinitionRepository = context.Locate.Advanced.GetInstance<GroupDefinitionRepository<AssociationGroupDefinition>>();

            associationDefinitionRepository.Add(new AssociationGroupDefinition { Name = "MyGroup" });

Hope this helps,

/Matt