I think "mata ai mashou" is "See u again", but u can say "Let's meet again". It depends on the situation. Basically, we don't use "mata ai mashou" with close friends cuz we say just bai-bai(=byebye) or jya-ne(=byebye). when we use "mata ai mashou(this is a polite sentence)" or "mata aou(ao-) ne", we might use such as flattery, so just saying even if we don't meet again.