Very difficult on a direct drive boat without adding a ton of weight. In addition to Berg's suggestions, what size of boards are the adults riding? If your riders are in the 160-200 lbs range for body weight you can cheat a bit by going with a slightly longer board, say, a 4'-8" (rather than standard 4'-6"). If your riders are +200 lbs then you need a big board and even then there's diminishing returns.

In my opinion, you have a boat that was designed primarily for skiing. Takes some work/compromise to get it to surf well.