Nickmerc's advice is right on the money.
The problem with having too many on board in 15-20 knots, especially if they're inexperienced, is that your ability to control trim is compromised. For example, you could have the very odd experience of sinking the stern if you try to tack and miss it. I've seen it happen. The weight aft is fine on a reach, but when you come into the wind, the bows rise up as the boat slows, the wind gets under the tramp, and the boat rotates one way or the other into the capsize. On the other hand, if you gybe, and the crew doesn't move aft and to windward quickly enough, you can easily go over. On a screaming broad reach, just a bit too much weight too far forward and you can dig in the leeward hull and pitchpole. All great fun, if your crew doesn't mind getting wet!
Your crew needs to be agile and willing to follow instructions quickly to move their weight where it's needed. I sailed my Hobie 16 for over 25 years, and learned that the key to keeping it upright is having the weight in the right place at the right time.