All of these sound very unnatural when you use the preposition "about". They would all sound more natural if they said, "I'm happy to know you." "I'm pleased to know you." "I'm glad to know you." (All of these, however, still sound just a tiny bit strange unless you are trying to tell that person that knowing them makes you a happy person.) However, if you're meeting them for the very first time, it would be more appropriate to say, "I'm pleased to meet you", even though you will sometimes hear people use "happy" or "glad" interchangeably. www.italki.com/garyalexander

Hi Evita ~ One thing, at least in American English, that makes your spoken sentences sound more natural, is dropping the "I'm." For example, if you've just been introduced to someone, say you met a friend while you were out walking, and your friend had someone you did not know with them, so the friend introduced the two of you. Here goes: Friend speaking: "Hey Evita, good to see you, this is my friend Roger. Roger, this is Evita." You: "Hi Roger, happy to know (or "meet") you." or, "Hi Roger, pleased to meet ("meet" is better than "know" here) you." or, Hi Roger, glad to know you." Any phrase will do, but drop the "I'm."