I want to translate the following sentence into French
The Martins invite me to their house.
Should I say
Je suis invité chez les Martin.
Je suis invité chez les Martins.
My tutorial book claims that I should use the former because "les XXXs" is used only to refer to a royal family, like "les Bourbons". But if I use "les Martin" here, it looks somewhat weird.
Can anyone help me? Best regards!