you can’t really pinpoint it down to one person. sports coaches and parents and everyone else in the world have been using this expression for a long time.

Granted – that was probably not your intent – but by asking the collective team for an answer you’ve discovered at least one of the meanings (I’ve always thought it meant a couple of things)

Any answer you (single i) could provide pales in comparison to what a group of individuals (lots of i’s = team) focused on the same goal can achieve.

As for who said it — go to google.com and use “there is no I in team” as your search string.