Is it correct to say Johnsons and Watsons say or Johnson's Watson's say?

Answer 1

If plural : Johnsons and Watsons

Example: "The Johnsons are having dinner with the Watsons tonight."

If plural possessive: Johnson's and Watson's

Example: "The Johnson's dog dug up the Watson's vegetable garden."

Both are correct depending on the context.

Answer 2

The Johnsons' house

Answer 3

"The Johnsons welcome you" is correct.