“Everyone” means “everybody” and is used when you want to refer to all the people in a group: “Everyone in my family likes spaghetti carbonara.”
But if you’re referring to the individuals who make up a group, then the phrase is “every one.” Examples: “God bless us, every one” (may each individual in the group be blessed). “We wish each and every one of you a Merry Christmas” (every single one of you). In the phrase “each and every one” you should never substitute “everyone.”
Paul Brians' latest blog post addresses the history of tendonitis, or at least the spelling of "tendonitis."