Traditionally the expression “just so happens” is used only with the subject “it,” with the word “so” providing emphasis: “Thank you for inviting me to your softball game, but it just so happens to be on the same date as my wedding, as you very well know since you are supposed to be my best man.” Expressions such as the following are popular but non-traditional: “I just so happen,” “she just so happens,” “they just so happen,” etc. In each of these cases, the “so” should be omitted.