For anyone thinking about joining a startup : be very, very careful before you accept a job offer. The main thing to consider is that you'll be working very closely with just a few people (from 2 to a dozen) for the foreseeable future. You'll have to really like these people as you'll be spending an awful lot of time with them.

When I was interviewing last, I was a lot more concerned with what the teams I met were producing than their culture, or how they interacted with each other. Long story short, I ended up working for a company with 10 employees, but I had never felt more alone in my entire life. Small things I took for granted at other jobs like eating lunch with people, sharing jokes, or any type of bonding felt near impossible.

I don't want to discourage anyone from working at a startup, but I feel it is very important that you consider the reality of working in that environment for a considerable amount of time.