you and I / you and me
Which form to use when I or me is joined to another person.
Use the form the pair requires. If the pair is the subject, use you and I. If the pair is the object of a verb, or the object of a preposition, use you and me. And does not change the form either pronoun would take alone. You and I can sound more refined, but after a verb or a preposition it is wrong: they billed you and I should be they billed you and me.
Examples
UseYou and I locked the gate. (subject)
AvoidYou and me locked the gate.
UseThey billed you and me. (object of a verb)
AvoidThey billed you and I.
UseThe decision is up to you and me. (object of a preposition)
AvoidThe decision is up to you and I.
To test the choice, omit the other person and the and. If what remains is correct, the full form is correct. I locked the gate is correct, so you and I locked the gate is correct. They billed I is not correct, so they billed you and I is not correct.