Strange topic maybe, but you know, rules are different in every culture. So,I guess you have already noticed there are only a few public toilets in France, and you must pay. It's because, in France it's nearly taboo !
So, when you are invited or even if you go to a restaurant, try (if it's possible) not to go to the toilet !! (I'm serious, you can believe me).
If you have to, don't go during the meal or during the apéritif, it would be regarded as very impolite. It's something you can do only with close friends. So if you really need to, go between the apéritif and the meal when everybody is moving and it's more discret, or go just before leaving.
Ask discreetly your host “excusez-moi, où sont les toilettes ?”.
Back to je suis invité / I'm invited