This will not be a big deal. All you need to say is something like this:
In [month year], when I was 21, I shared some beer with my girlfriend and her friends who were 19 and 20 years old. My Resident Assistant noted this in my student conduct record. That record has since been expunged. I realized that I was not behaving in a responsible manner to allow people under 21 access to beer and I have refrained from doing so ever again.