How to prepare soup for an office potluck?

As suggested in comments (and in the USDA quotation below), the best option if you are cooking the soup ahead of time would be to cool it rapidly, then bring it to work and reheat it with a microwave if one is available. Then add it to the slow cooker to keep it hot until serving time. Most slow cookers are not equipped to reheat food within a safe timeframe.

More details: Most crockpots and slow cookers contain advice in their instructions explicitly against reheating food in them. Major food safety organizations say similar things (see here for example). Instead, the USDA says:

Reheating leftovers in a slow cooker is not recommended. Cooked food should be reheated on the stove, in a microwave, or in a conventional oven until it reaches 165 °F. Then the hot food can be placed in a preheated slow cooker to keep it hot for serving—at least 140 °F as measured with a food thermometer.

Soup is usually a good growth medium for bacteria. And even soup that is boiled is not usually completely sterile: some bacteria can survive very high temperatures, often by going into a dormant state (e.g., a "spore" form) but then can reactivate when the soup is cooler and grow again. That's why commercial food codes (like the FDA) have very strict regulations on cooling and reheating. While those guidelines vary depending on the exact situation, one thing that is required for commercially reheated food is that it be heated to at least 165°F within 2 hours. Most crockpots and slow cookers will not be able to achieve that with any significant quantity of cold food.

And that part of the food code is dependent on code-compliant practices for rapid cooling after initial cooking, as well as storage. Presumably you will be transporting the soup to work, during which time it will also be unrefrigerated, which removes some time from that 2-hour clock as well.

For consumer recommendations, the FDA and USDA have a general procedure that you don't tend to want cooked food to remain in the temperature range from about 40°F to 140°F for more than 2 hours total. And here "total" would mean adding both the time to cool the soup AND to reheat it. It's very unlikely you could cool soup (even in an ice bath) and then reheat it to 140°F in a slow cooker all while spending less than 2 hours in that temperature range.

Some may see this as being overly cautious [See NOTE below], but the FDA has regulations for commercial cooking to avoid making large numbers of people sick. I'd be extra cautious and follow the same recommendations that commercial kitchens follow when serving lots of other people.

NOTE: Now, I know some who read this will say, "I reheat food all the time in my slow cooker and never got sick." Okay. But first consider that many instances of food poisoning are minor gastrointestinal distress that may not appear for a couple days after you've eaten something that was off. If that happens, you may be lucky that it wasn't worse. Most cases of food poisoning luckily are minor, but they tend to happen more frequently than most people think.

But even if you're willing to take the chance for yourself at home, consider the unlikely scenario where a cooked dish makes a bunch of coworkers ill. I personally wouldn't take that risk.