It means "I am happy to have done X". As a reply to "It's my pleasure to have you on my friends list", it means that you are also happy to have that person on your list. While it can be used after meeting with someone, it can also be used after giving help and the person thanks you. It's similar to "You're welcome". For example: - Thank you so much for helping me with organizing the meeting today! - The pleasure is/was all mine.

It means "you're welcome."

When someone says "the pleasure is all mine" it is generally said when you are first meeting someone or when you are thanking someone. For example: One person will say "It is nice to meet you" and the other will reply "The pleasure is all mine." It is a common reply which is basically like them saying "While it was a pleasure for you, I enjoyed it more"

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Yes you can use it as a response for the above-mentioned phrase. "The pleasure is all mine" is usually said in response to "I'm pleased to meet you". It means something like "I'm even more pleased than you are.