It's not uncommon for a couple of people to walk out of almost any screening. "Blind" recruited screenings can have higher walkout rates, since people don't know what they're seeing ahead of time and even if the movie is good, it still might not be everyone's cup of tea from a genre perspective, etc. When people leave, someone from the research vendor will ask them why they're leaving, just to have a sense of whether it's related to something specific in the movie. Walkout rates and reasons for leaving are always reported back to the client that's conducting the screening.
In terms of focus groups, typically 20-25 people are seated for the discussion after everyone is done with their surveys. While people are checking in and lining up, there are typically a couple of people from the research vendor who are interacting with the people lining up, getting a gauge on whether they seem talkative, willing to share their opinions, etc. Usually, the research vendor will try to get 25-28 people who say they are willing to stay and participate in the focus group, since there is always a little bit of a drop-off after the screening for various reasons (time, changed their mind, really hated the movie, etc.).
Then, depending on the research vendor, sometimes they'll check in with the folks they had pre-screened for the focus group to get a gauge on how they rated the movie, just to make sure that the group is reasonably representative of how it looks like the audience overall rated the movie (meaning, you don't want the focus group to be too negative or too positive, compared to what the overall scores are going to be).