Just write The Johnsons
The stock symbol for Johnson Controls is JCI and it is traded on the New York Stock Exchange.
The Johnsons' house
Aaron Johnsons mum is called Aaron Johnson.
Two Johnsons have been US presidents: Andrew Johnson and Lyndon B. Johnson.
"The Johnsons welcome you" is correct.
Andrew Johnson, Lyndon Baines Johnson, Samuel Johnson ( Mark Twain)