Actually, it has nothing to do with the original team. It's from the SETI project.
>From 37Signals' company manifesto
"Mankind constantly analyzes radio waves from outer
space in the search for extraterrestrial intelligence. Since this
analysis started, almost all of the signal sources have been
identified. 37 signals, however, remain unexplained."
Ian
------
They named their company 37Signals because "37" was their original team
size. I guess, I remember reading it on their blog or elsewhere.