Inspired by this question about an outdoor minyan, I am curious, what are the requirements for counting Jews in an outdoor zimun? Say three Jews who otherwise would qualify in a zimun ate together outside, what are the requirements for counting them? Is it similar to a Minyan with regard to having to see and hear them?
See the halachipedia article on zimmun. The important part would seem to be independent of whether you are indoors or outdoors and is:
There is only an obligation to have a zimmun if three or more people eat together. The three conditions to be considered “together” is that those who are eating
eat while seated,
sit at one table, and
start or end the meal together.  They are considered starting together if they all start to eat the first kezayit of bread while the others are still eating that first kezayit.