Purchased OPS Coventry storage bench which I love. Now find the load limit is 250lbs. Using as dining bench and need to increase load limit for two persons at least 500lbs.
the only place to (inobustrusively) re-enforce that is from underneath the seat.
The legs are fairly thin and contoured, but most force on them will be vertical so they should be okay. Due to the contouring you would need to shape any bracing to match - this is (probably) beyond you at this point.
The underneath could be braced though with some 2x4's - cut to length and push hard up against the underneath of the slats (or even a bit of ply, cut to size).
The ends will need to be secured to the sides of the bench - this could be done easily with a few 2" or 3" screws in each side, or less visibly with a brace (bit of wood) under each slat fixed to the side. The weak spot is always going to be those legs though.
However..... I suggest you get your 2 x 250lb pound friends and get them to sit on the bench as it is and see how it handles it. I suspect that unless you are planning on dancing/jumping up and down on the seat, that it would support your weight fine.