Hello Anon
In this sentence –
1. Say hello to John from Rachel and I.
– 'Rachel and I' are not subjects. They are objects of the preposition 'from'. You can test this by inserting 'from' in front of each of them:
2. ?Say hello to John from Rachel and from I.
This sentence is wrong, so 'I' must be wrong. Instead, we must say:
3. Say hello to John from Rachel and from me.
This reduces to:
4. Say hello to John from Rachel and me.
MrP