When food is being served and has an appetizing aroma or a pungent taste, a morsel should be given immediately to anyone standing around. Otherwise the person could come to sakkana. (MB 1,2,3 – BeH “lashamash”)
Minhag Yisroel is to invite anyone to eat if he enters when you are in a meal. (BeH “lashamash”)
It is dangerous to swallow saliva that comes to your mouth because of an appetizing aroma when you aren’t able to eat the food. The saliva should be evicted from your mouth. (MB 3)
Some poskim are machmir not to give bread to a person if it seems like he will eat it without netillas yodayim. You can be maykil if you give him the bread for tzedoka. (MB 9)
If you aren’t certain that a poor person will say a beracha on food that you give to him, you can still give it to him for tzedoka. However, if you know that he will not say a beracha (out of wickedness), it is forbidden to give him food. (Rama 2 – MB 11)
You are allowed to give food as tzedoka to a poor person who isn’t able to say a beracha. (MB 11)