בChelek 2
- When two people finish eating and along comes a third person who is interested in combining for a zimun, he can, according to the following conditions:
- The two could theoretically eat a little more if offered their favourite food.
- The third person sits at their table and is given a k’zayis of food to eat (or a r’viis of drink, other than water).
- The two have not yet verbalized “Let’s bentch” nor washed mayim achronim. (ShA , – MB , , – BeH “k’zayis”) L’chatchila, offer this third person a k’zayis of bread (this doesn’t apply to the tenth person if there are nine people). (MB ) One person can combine with 9 others, two with 8, and 3 with 7 according to the above conditions and they can bentch with the Shem. Just make sure the leader of the zimun is one of the people who ate bread. (ShA )
- The people combining to the zimun must make sure to say their own appropriate beracha achrona after they finished listening to Birkas Zimun. (ShA –MB )
- Do not be motzi anyone in Birkas Hamazon if you yourself aren’t obligated to bentch. (MB )
- Some poskim hold that you aren’t obligated in Birkas Hamazon d’oraisa if you didn’t drink and you feel thirsty. (Rama )
- If your stomach is full before you eat to the point that any more food would be disgusting to you, the food isn’t fit to bless upon, not before nor after – it is not called “eating”. (MB )