Coming from someone who just went through Beast (or, well, last summer!), I wouldn't worry too much about it. I don't think they ever disqualify a kid for being underweight on R-Day (unless you are SEVERELY under minimums, to the point where Beast could be a detriment to your health). If you're on the cusp of the BMI or weight minimums, you should be fine. However, it definitely WILL NOT hurt you to bulk up a little bit before Beast. A lot of kids report losing weight during CBT, so if you're already underweight, I would consider putting effort in to gaining weight.
Hope this helps!