Who can answer this English grammar question about the difference between past perfect&past simple"?

[SirGeorges]

Who said anything about leaving school man?

Take the example I gave you before:
By the time I arrived(past simple), everyone else had left(past perfect)
^This sentence means that everyone left at 10:00 and I arrived at 10:15
So the verb in past perfect happened FIRST [:everybody had left]
And the verb in past simple happened SECOND [:then I arrived]

In YOUR sentence, if the time sequence is like
1.First you used to be partners at stanford
2.Then he got a job at Microsoft
3. After that he left that job to join you in a new company
By using past perfect you show that he used to be a partner of yours at Stanford, but then ceased to be one.
He then got a job at Microsoft.
And after that he quit to start with you a new company.

If the whole time sequence thing is too much for you, why don't you try re-writing that sentence into smaller sentences each one in a different tense?