Pros
Young company, rapidly growing, smart people, teamwork oriented.
If you have great ideas and are willing to roll up your sleeves, this is the place for you.
Management encourages you to understand the industry and the business overall, rather than just doing your job in a vacuum.
You get to help create the foundational framework of the business.
Cons
This could be a pro or con depending on your personality and work style: processes are still being fine-tuned and may shift based on the needs of the business, so you'll need to adapt quickly.