At a kabalat panim, would the baal hasimcha have to make a new shehakol or adamah etc. everytime he went in and out of the hall – or do the other people participating koveah his brocha?
In principle, the concept of somebody remaining in a room being “kove’a a berachah” is said with regard to a proper meal, and it is hard to apply it to a simcha, where people are not kavu’a at all.
However, there is room to argue that the entire presence of the ba’al simcha is not kavu’a, because it is the way of ba’alei simcha to go in and out the whole time. In this case, it is possible that the one berachah he makes at the beginning will apply throughout, even after he goes out and comes back in. Please see here for details of this halachah.
It is preferable, however, to eat bread, which will alleviate any doubt over this matter.