Hello,
Thank you for your message. I'm actually going to pass this to English to French, as I'm not sure témoin should be the translation for groomsman.
@DearPrudence, what do you think? At UK weddings, the witnesses fulfil a legal function, I assume it's the same for témoins in France. They are the people who sign the register to say they have legally witnessed the wedding. It's possible that a groomsman might be a witness too, but equally possible that he might not. It's basically the male equivalent of a bridesmaid. He's there to help the groom get organised, make sure everything runs smoothly.