Dear Anjali,
As far as the HR activities are concerned, there is no difference between a full-fledged company and a start-up. In fact, while working at a start-up, the responsibility of the HR professionals increases as they need to establish systems and processes in the company. The strength of the systems and processes fosters the growth of the company.
The most important thing that rests on the shoulders of the HR professionals working in a start-up is the introduction of the measurement of productivity measures for each department. In many organisations, this fundamental job that should have been done when the organisation is in the primitive stage is not done and they pay price for it. But when the realisation dawns upon them, it is too late.
Lastly, the HR professionals of start-ups cannot work in isolation. They need support from the top leadership as well. The start-ups work on a shoestring budget. But then they develop a mindset that "We will do ______ when our organisation becomes big. Right now we do not need this". Many startups fail because of the tendency to give self-certificate or living in self-denial. Therefore, treat yourself as if you were a 10,000-employee company.
Thanks,
Dinesh Divekar