Making a last name into the plural form might be one of the most common grammatical errors one comes across. It's especially noticeable during the holiday season when the mailbox fills with cards. But this topic is an important one no matter what time of the year. I can honestly say that after being in the stationery business for over 20 years, I have had clients push back about using an apostrophe in their name when wanting to make it plural and I have had to work hard to explain to them why they should not do it.
You can refer to the image for a quick reference guide but here are the details:
Most last names can be made into the plural form by adding an S. So Smith becomes Smiths, Miller becomes Millers. Even a name that ends in Y just gets an S, so Fry becomes Frys. You don't change it to the plural form like a noun, example sky becoming skies.
For names that end in the letter S, you just add an ES to make their name plural. We are inviting the Williamses over for dinner this weekend.
For names that end in X, CH, SH and Z, you also add an ES to make their name plural.
When in doubt, if you don't know how to address someone, just take their name and refer to them by their family name, so you can speak about The Miller Family if you are worried about whether you should be referring to them as the Millers or the Miller's (remember, apostrophes are never, ever needed when it comes to the plural form of a name)