You need only two cups, water, and an opaque fluid. They can perform the following in front of each other and with items very likely already at their dinner table.
Put a small amount of water in one cup. Put a small amount of opaque fluid in the other cup. This is the pollutant that indicates a no vote. The first voter takes both cups, with the water in the left hand and the pollutant in the right, and votes under the table. He takes 10 seconds exactly regardless of vote, pouring the pollutant into the water for no or doing nothing for yes, making sure to disguise his vote by clinking the cups together repeatedly. He then passes the cups under the table to the next voter, making sure to put water in left and pollutant in right. This continues to the last voter, who then reveals the cups after his 10 seconds.
The result will be immediately clear (or opaque, lol) and only one's word could be used to determine any vote.
Am alternative using only water and two cups:
Water is put in both cups, but instead of pouring one specific cup into the other, a no voter pours one to the other and the other back to the first, thereby mixing both cups. A unanimous yes reveals water in two cups, but any number of no votes reveals water in only one cup.