1-I met John at a party last week.
Does this mean:
a-I was introduced to him.
b-I talked to him for the first time. I knew him but had not talked to him.
c-I knew John from before and I happened to see and talk to him at a party.
d-We knew each other and had arranged to meet at that party.
Could one use "1" if John was one's friend?
Met is a pretty general verb. All four of your interpretations are possible and correct, and there are more. To meet someone is simply to encounter that person.
Your doubt about a- is also correct. Logic, not grammar, dictates that you cannot be introduced to somebody you already know.