Craft\ImportService::prepUsersFieldType PHP Method

    private function prepUsersFieldType($data, FieldModel $field)
    {
        // Get field settings
        $settings = $field->getFieldType()->getSettings();
        // Get group id's for connecting
        $groupIds = array();
        $sources = $settings->getAttribute('sources');
        if (is_array($sources)) {
            foreach ($sources as $source) {
                list(, $id) = explode(':', $source);
                $groupIds[] = $id;
            }
        }
        // Find matching element in sources
        $criteria = craft()->elements->getCriteria(ElementType::User);
        $criteria->groupId = $groupIds;
        $criteria->limit = $settings->limit;
        $criteria->status = null;
        $criteria->localeEnabled = null;
        // Get search strings
        $search = ArrayHelper::stringToArray($data);
        // Ability to import multiple Users at once
        $data = array();
        // Loop through keywords
        foreach ($search as $query) {
            // Search
            $criteria->search = $query;
            // Add to data
            $data = array_merge($data, $criteria->ids());
        }
        return $data;
    }