To extract the field value in Drupal 8

field_value.php

// We can go the long way
$field = $group->some_field->first();
// then
$reference_entity_id = $field->getValue()['target_id'];

// But there is a better way to do it
$reference_entity_id = $group->some_field->target_id;


// The same way we can extract simple types, like string and integer
$field_value = $group->some_field->value;