Shevuos, Chapter 4, Mishnah 10. This mishnah teaches that the oath of testimony requires the one administering the oath to identify the people he is placing under oath. A general statement directed at a large group of people is not enough, and the mishnah brings an example of this.
The words of the mishnah:
"Amad b'veis ha'knesses v'amar" - a person stood up in the synagogue and addressed the congregation as a whole.
"Mashbia ani aleichem" - I hereby place all of you under oath.
"She'im atem yod'in li eidus she'tavo'u u'te'iduni" - that if you know testimony concerning my case, you should come and testify for me.
"Harei eilu peturim" - they are not liable for the oath of testimony.
Even if there were two people in the congregation who did know testimony for him, and afterwards they denied it and claimed that they knew no testimony for him, they are exempt. The reason is that they were not identified specifically, and the one administering the oath did not place them under oath directly, but rather addressed the entire group in a purely general way.
When the address is directed at identified people:
However, if he said, even in front of a large group, "I am speaking to you," "all of you standing here, if you know testimony for me you should come," this is a full oath. It makes no difference whether we are dealing with two people or with a large congregation; what matters is that the one administering the oath identifies those standing before him and directs his words to them explicitly. A general appeal to a large crowd, without identifying anyone, is not considered an oath of testimony.
In summary: the oath of testimony takes effect only when the one administering it identifies the witnesses and directs the oath to them directly. A general oath imposed on a congregation in the synagogue exempts even those who know testimony and later deny it, whereas an explicit address to those standing there creates liability, no matter how many of them there are.