Array in meta key?

A postmeta value can be an array. You save it the exact same. Updating/Getting postmeta will detect if the value is an array/object, and serialize the value so it can be stored in the database.

// Saving
$users = array( 5, 20, 25, 29, 30 );
$saved = update_post_meta( get_the_ID(), 'registered_users', $users );

// Getting
$users = get_post_meta( get_the_ID(), 'registered_users', true );
// array( 5, 20, 25, 29, 30 )

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