Is it grammatically incorrect to make a surname plural, without making it possessive? Or is it merely informal? (For example, "The Smiths are coming along, too," or "The Petersons are a very pleasant family.) Most people use this sort of language all the time. I've been studying grammar for years, but never have I come across a rule that states this as either incorrect or correct.
You can absolutely pluralize surnames without making them possessive. Apostrophes aren't needed in pluralization without possession. When you pluralize a name, you are pluralizing a noun. Most of the time, rules for pluralizing common nouns work the same way for proper nouns. (Add an -s or -es as is appropriate.) Example 1: There are four people in the Smith family. Together, they are the Smiths. Example 2: There are four people in the Jones family. Together, they are the Joneses.
If a name ends in a vowel, the rules can be a little bit different than with common nouns. Just remember that you should never change someone's name. You add to it to pluralize it, but the root (the name) remains unchanged. (Don't change -y to -ies, etc.) Example 1: The Rosetti family becomes the Rosettis. Example 2: The Murphy family becomes the Murphys.