Graham,

I would do some rehearsing with a some coworkers if you have the time between now and the CEO tour.

Scout out the candidate locations at the same time of day. Lighting changes fast during the golden hour so you have to be flexible. Tour agendas get delayed. You may be taking the photo when it's starting to get dark. And as you mention, there's the weather. I'd have an indoor location as a back-up.

If you do have the benefit of the low sun you definitely don't want them facing into it. So you will need some fill to lighten the shadows. I haven't tried a solo SB600 with such a sized group before and with direct low sun. But I'm sure it will help. With some luck the light will be filtered by thin clouds or haze.

You may be able to do better by getting the SB-600 off-camera and use the built-in as master. If the sun is not filtered it may still be bright enough to cast harsh shadows so you'll need to make the most of your two flashes. Mount the SB-600 on an umbrella stand and set it up high. (don't use an umbrella unless it's getting dark. The umbrella may lose too much light.) Set it to the shadow side of the group and move it in closer to the group so it's just off frame. Make sure the IR sensor sees the camera's flash.

Your flash will be most effective as a fill at the fastest normal sync speed. For the D300 this is 1/250s. I don't know if you'll have to go this fast to get the proper fill ratio but going any faster with FP sync is counter productive. Set the aperture to get the desired DOF and complete the ambient exposure with ISO.

Get a small rehearsal group together to outline the actual expected group. You'll need enough to create a center focus target and to represent the depth and breadth of the group. It will help test out your fill flash set-up, DOF etc.

DOF will depend on what relationship the group has to its background. You might want to go with greater DOF if you want the company building in the background. Otherwise I'd try to soften the focus on the background.

If you have stairs to place the group upon that helps. Otherwise a step-ladder for you is a good idea.

If the light is direct golden hour sun it's desirable to put a 1/2 CTO gel over your SB-600. This will better match the color temperature of the flash to the sun. Otherwise the filled shadows will look blue compared to the sunny side. Don't sweat it if you can't get such a gel on short notice.

I have a 28-70mm that I use for my large group shots. I find that 28mm is a good focal length for many large groups on DX. Go much wider and the edges of the group may start to show a bit of stretching.

Check for any weaknesses of your lens on the rehearsal group. There should be someone in each corner of the expected group size. If your lens shows too much aberration or distortion on these folk don't frame the group so tight. My 28-70mm has a centering defect that causes the extreme left edge to turn to mush.

If you have to fall back on the indoors, a second remote flash would help. But I'd recommend both remotes to use an umbrella to diffuse the light since the flash is now your primary light.

Can the company help you with the extra equipment purchase? The extra flash, umbrellas and stands. (They are saving themselves a lot of expense not having to hire a professional.)

Alternatively you can use bounce flash off a low light neutral colored ceiling. This will help diffuse the light. I do this a lot with good success. If you have a D300 you may have to turn up the ISO quite a bit. This will make the most of the bounce and any ambient light. But the ambient plus flash may be mixed. If you bounce, secure a small white card to the back of the flash head with a rubber band. That will direct some light directly forward to the group and create catch-lights in their eyes. It will also redirect much of that back directed light that would otherwise be wasted. With bounce I find I have to often adjust the flash compensation. So practicing with the rehearsal group will help.

It's best to shoot raw, I can cover up a lot more of my mistakes that way. (WB and exposure.)

Ohhh almost forgot. You're part of the group right? Don't forget to work yourself into a few of the photos.

Good luck.
Steve