_config.yml

Injector:
  ManyManyList:
    class: StrangersClub\ManyManyList

ManyManyList.php

<?php

namespace StrangersClub;

use DataList;
use ManyManyList as SilverStripeManyManyList;

class ManyManyList extends \ManyManyList {

   /**
     * @param mixed $item
     * @param null $extraFields
     * @throws \Exception
     */
	public function add($item, $extraFields = array()) {
		$foreignIDs = (array)$this->getForeignID();
		$joinTable = $this->getJoinTable();

		// Ensure we have an object to work with
		if (is_numeric($item)) {
			$item = DataList::create($this->dataClass)->byID($item);
		}
		
		if ($item instanceof $this->dataClass) {
			$item->extend('onBeforeManyManyRelationAdd', $foreignIDs, $extraFields, $joinTable);
		}

		parent::add($item, $extraFields);

		if ($item instanceof $this->dataClass) {
			$item->extend('onAfterManyManyRelationAdd', $foreignIDs, $extraFields, $joinTable);
		}
	}

	/**
     * @param int $itemID
     * @throws \Exception
     */
	public function removeByID($itemID) {
		parent::removeByID($itemID);

		$item = DataList::create($this->dataClass)->byID($itemID);

		// Just in case we're given an invalid ID
		if (!$item instanceof $this->dataClass) {
			return;
		}

		$foreignIDs = (array)$this->getForeignID();
		$joinTable = $this->getJoinTable();
		$item->extend('onAfterManyManyRelationRemove', $foreignIDs, $joinTable);
	}

}

MemberExtension.php

<?php

class MemberExtension extends Extension {

	private static $belongs_many_many = array(
		'BookedEvents' => 'Event'
	);

	/**
	 * @var array
	 */
	protected static $join_table_relation_mapping = array();

	/**
	 * ManyManyList doesn't keep track of the relation name being edited, only the join tables/fields,
	 * so this is used to build a mapping from join table -> relation name
	 * 
	 * @return array
	 */
	protected static function get_join_table_relation_mapping() {
		if (empty(self::$join_table_relation_mapping)) {
			$obj = singleton('Member');
			$manyMany = $obj->manyMany();

			foreach ($manyMany as $relation => $targetClass) {
				list($dud, $dud, $dud, $dud, $joinTable) = $obj->manyManyComponent($relation);
				self::$join_table_relation_mapping[$joinTable] = $relation;
			}
		}

		return self::$join_table_relation_mapping;
	}

	/**
	 * @param array $foreignIDs
	 * @param array &$extraFields
	 * @param string $joinTable
	 */
	public function onBeforeManyManyRelationAdd(array $foreignIDs, &$extraFields, $joinTable) {
		// Map the join table to a human-readable relation name
		$tableRelationMapping = $this->get_join_table_relation_mapping();
		if (!isset($tableRelationMapping[$joinTable])) {
			return;
		}

		// If this is new event being added to/removed from the 'BookedEvents' relation...
		if ($tableRelationMapping[$joinTable] === 'BookedEvents') {
			// Ensure the payment date is set
			if (!isset($extraFields['PaidDate']) || !$extraFields['PaidDate']) {
				$extraFields['PaidDate'] = SS_Datetime::now()->value;
			}
		}
	}

}