See Leviticus 4:13
"When the entire congregation errs..."
Hazal equate the following :
- "The entire congregation errs" ==> The Sanhedrin errs.
Now, In the absence of any Sanhedrin it makes sense to go back to the original text. And it follows now to imply the converse - Viz.
- The Sanhedrin ==> The entire Congregation of Israel.
In the USA Congress - or in Canada the Parliament - Represents all of the people. In the absence of such a representative body then all of the people represent themselves.