Writer chose the dumbest ending possible
I see the consensus is that the ending was great. I don't see it. It wasn't smart or thoughtfully planned out. It was just a cliche. Leave the audience in the dark and let them make up their own minds. There is nothing smart or clever about that. But that's fine. It was really the only option. The movie was so far out there (in a good way) that having some sort of normalcy restored by a last minute rescue or something along those lines would have been out of place. Therefore, you know it is going to have to end with a huge question mark. The writer didn't have much to work with in terms of an ending, but he ended it in the dumbest fashion I can think of.
Let's throw a curveball and have Glen turn out to be a nutcase. That'll be clever because all of Tracy's boyfriends turn out to be crazy. We'll have him mix up some rat poison and then somehow convince all the people who aren't insane to go Jonestown with him. Awful.
How about skipping the whole part about Glen turning out to be crazy and just end the movie with the friends finally seated at the table talking and eating..finally having brunch. Pan out, movie over, ending left up to the audience.