1985 Islander 34

How many people can I legally take a board for a day trip?

I’m not aware that there is a legal limit on number of persons for the boat in question. Load capacity deals in weight, not numbers. This means adding four people weighing 250 lbs will have the same effect as adding 7 people weighing 145 (in this example, lowering the boat in the water about one inch).
The only legal issue I’m aware of regarding passengers and crew is that you need a PFD (life preserver) on board for each person (there might be other legal issues if you are planning on charging your passengers).
Hope this helps.