Why do you not add an s to retourné when you say - vous y êtes retourné?

Hi Catriona,

Without the context it is hard to tell.

Vous can be singular or plural...

Vous y êtes retourné refers to a single person in formal "you". 

Vous y êtes retournés addresses a group of people, hence plural and hence retournés. 

Thanks for pointing this out. I have always been adding the s to the pp when using être as the auxiliary verb and the vous form regardless of whether singular or plural. Oops!