a ustedes
pron.
to you
a ustedes
= you
Ex: I am glad of the opportunity to discuss this subject for several reasons: firstly, I have been interested in it for some time and would like to share some of my thoughts with you.
a vosotros
= you
Ex: I am glad of the opportunity to discuss this subject for several reasons: firstly, I have been interested in it for some time and would like to share some of my thoughts with you.