_config.yml

Injector:
  ManyManyList:
    class: MagicManyManyList

MagicManyManyList.php

<?php

class MagicManyManyList extends ManyManyList
{
    /**
     * @var array
     */
    protected static $join_table_relation_mapping = [];

    /**
     * Returns the name of the relation being edited
     *
     * @return string
     */
    public function getManagedRelation() {
        if (!array_key_exists($this->dataClass, static::$join_table_relation_mapping)) {
            static::$join_table_relation_mapping[$this->dataClass] = [];

            $obj = singleton($this->dataClass);
            $manyMany = $obj->manyMany();

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

        $map = static::$join_table_relation_mapping[$this->dataClass];
        return isset($map[$this->getJoinTable()]) ? $map[$this->getJoinTable()] : '';
    }

    /**
     * {@inheritdoc}
     */
    public function add($item, $extraFields = [])
    {
        $foreignIDs = (array)$this->getForeignID();
        $joinTable = $this->getJoinTable();
        $relation = $this->getManagedRelation();

        // 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', $relation, $foreignIDs, $extraFields, $joinTable);
        }

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

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

    /**
     * {@inheritdoc}
     */
    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();
        $relation = $this->getManagedRelation();
        $item->extend('onAfterManyManyRelationRemove', $relation, $foreignIDs, $joinTable);
    }
}

MemberExtension.php

<?php

class MemberExtension extends Extension
{
	private static $belongs_many_many = [
		'BookedEvents' => 'Event'
	];

	/**
     * @param string $relation
     * @param array $foreignIDs
     * @param array $extraFields
     * @param string $joinTable
     */
	public function onBeforeManyManyRelationAdd($relation, array $foreignIDs, array $extraFields, $joinTable)
	{
		// If this is a new event being added to/removed from the 'BookedEvents' relation...
		if ($relation === 'BookedEvents') {
			// $eventsJustAdded = Event::get()->byIDs($foreignIDs);
			// Do stuff, send notifications, whatever
		}
	}
}