(PECL solr >= 2.2.0)
SolrQuery::addGroupField — Add a field to be used to group results.
The name of the field by which to group results. The field must be single-valued, and either be indexed or a field type that has a value source and works in a function query, such as ExternalFileField. It must also be a string-based field, such as StrField or TextField Uses group.field parameter