Is it "me", or is it "I"?

Hello, and thank you to all who have had a part in making this site possible. I'm so glad that you are all here to help me answer my question.

I will have to share the background of my question before I feel comfortable soliciting answers. So, please, bear with me.

Early one evening, sitting in a bar, my friend informs me of a wager he has with the bartender. Aparrently, she had asked another customer about the cash lying on the bar, not sure whether it was a tip or not. The question that she asked was, "Is that for Megan and I?"

My friend corrected her, and the debate began. I agreed that the first person pronoun that the bartender had used was incorrect in this instance.

I attempted to explain why the question should have been either: "Is that for Megan and me?" or "Is that for me and Megan?". But my friend interrupted and said that the reasonning was as simple as the fact that she was asking a question, not making a statement.

This is where I disagree with my friend. If the bartender had stated "This is for Megan and I.", she would still be incorrect. The proper pronoun to use would still be "me".

That is where my argument stops, because I do not know exactly the rules for such things. I do know that in many instances, "I" is the correct pronoun. For instance: "I went to the store." instead of "Me went to the store."

Is this because of the type of statement: one is declaring posession; one is describing an action?

Please help us. I would like to know which of the initial questions is correct, as well as the rules and reasonning to back it all up.

That way, when my friend has to pay me for being wrong, he can be reimbursed by the bartender's payment.

Thank you in advance for all of your input.

Sincerely,
Crissy