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
SolrQuery::setGroupLimit() - Specifies the number of results to return for each group. The server default value is 1
SolrQuery::setGroupMain() - If true, the result of the first field grouping command is used as the main result list in the response, using group.format=simple
SolrQuery::setGroupNGroups() - If true, Solr includes the number of groups that have matched the query in the results
SolrQuery::setGroupTruncate() - If true, facet counts are based on the most relevant document of each group matching the query