On ne s'était jamais autant ressemblés

Question

On ne s'était jamais autant ressemblés que ce Noël-là,       - ou     On ne s'était jamais autant ressemblé que ce Noël-là, ?

Accepted Answer

Bonjour David,
Chris is correct and his examples are excellent! ;-)
Keep in mind that, nowadays, in the majority of cases, the past participle (with "on" as a subject) will agree in the plural form because "on" usually refers to very precise people. 
Here is a link to a partner site: Agreement with on
And another very interesting one (albeit in French) about "on": Questions sur le pronom « on »
I hope this is helpful.
Bonne journée !

Answer

On ne s'était jamais autant ressemblés... -- We never looked so much alike...

Answer

But 'on' is singular - why not ressemblé ?

Answer

The pronoun on exhibits some strange behavior when used as the plural "we": the verb is 3rd person singular (était) but the participle matches the intended meaning. Here some examples highlighting the difference:
On est allé à la lune. -- We went to the moon ("mankind" went to the moon).On est allés au cinéma. -- We went to the movies (mixed gender or male group)On est allées au cinéma. -- We went to the movies (female group)