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.