It was interesting - I was pulled into a startup by a friend, and I was the first full-time employee - he joined later, and we were peers - he headed up HW and I led the SW and architecture side - one of the execs was also known, as we worked years back at the same company.

It comes down to the dynamics - if one is asked to join in, it's because one has demonstrated capabilities and the ability to execute in a team environment.