list of deartments I can think of in a big company:

sales,marketing,software development,legal,payroll,hr,accounting,distribution, investor relations,manufacturing/service offering,desktop support,server support,customer service,inventory,recruiting.

Alot depends on the company and the founders. At a minimum you need to focus on sales and marketing and probably an MVP or service that you can automate. If one of t he founders is an expert in XYZ I don't see a problem in them do XYZ for the company but if your not selling or building the core product you may be wasting time.

Accounting. Legal. Everything that isn't central to providing or producing value.