Say hello to John from us.
Say hello to John from Susan and I.

This is a very simple matter. Which would you say: "Say hello to John from I" or "Say hello to John from me"? The second one, of course. When you add another person's name, you do not need to change the pronoun case.
The reason people always think they should say "[name] and I" is because a great deal of fuss has been made to eradicate the incorrect sentence, "You and me are walking to the store." And that sort of thing. You would say, "I am walking to the store," not "Me am walking to the store"; therefore you need to say, "You and I are walking to the store." But just because "I" is correct in this case doesn't mean you use it automatically.
A long-winded technical explanation of all this:
The different forms of a pronoun (a word like "you" or "me" that replaces a noun) are called "cases." When a noun or pronoun is the one doing the action to someone else, it is said to be the "subject" of a sentence; for example, if I were to say, "I threw the ball," then I am the subject of the sentence because I am the one doing the thing. The ball is having something done to it; it is called the "object" of the sentence.
Prepositions (little words like "to" or "on" or "under" or "between" that go before nouns and pronouns and connect them to other words) also take objects, just like verbs. Take this sentence, for example:
"I threw the ball at Joey."
"I" is the subject, the one doing the throwing; but there are now two objects. "Ball" is the object of the verb, the thing on which the action is being done; and "Joey" is the object of the preposition, the noun being connected to the rest of the sentence. Does all this make sense?
Anyway, if you understand the distinction between a subject and an object, you will have an easy time deciding which pronoun to use. Pronouns such as "I," "he," "she," "they," and "who" are the types that you use for the subject of a sentence; pronouns such as "me," "him," "her," "them," and "whom" are the ones you use for an object. In your sentence, because of the presence of the preposition "from," you need an object rather than a subject; therefore you must choose "Rachel and me" instead of "Rachel and I." If you learn to identify the parts of a sentence and how they relate to each other, this sort of thing becomes second nature.
But yeah, if all that is too complicated, just go with your ear. Your ear will tell you that "from me" is correct, and therefore "from Rachel and me" is also correct. But I usually recomment understanding the grammar behind it for two reasons: a) if you rely exclusively on your ear to guide you all the time, you are groping in the dark, and you will never be quite sure of what you are doing (a thoroughly unpleasant feeling, and plus, you'll be prone to bad advice); and b) sometimes your ear will be wrong, since your ear picks up what it hears around it, and if the people you talk to are also frequently wrong, your ear will not be reliable.

Anyway, hope this helped. If any of this is unclear, just let me know what is confusing you, and I'll try to explain in a simpler way.