members dining at the house all at once?

Typically eating hours last 1hr and 30 minutes to 2 hours. That allows the amount of girls to be dispersed. Also some girls don't eat at the house depending on their schedules.

It's mostly come and go. Girls grab plates to go on their way to meetings, and a lot of girls are doing homework and will eat in the basement or in their room upstairs. There's still a lot of girls to sit an eat with, but not 400. You won't even have all 400 together at chapter. Girls get excused for sports, meetings, class, etc.

Well, MOM, and you are a mom because no student would care - it's not like when we were in school. (Yes, I'm a mom too). Back then you would all go into the dining area at once. Someone would pull out the housemother's chair for her to be seated, and then everone else sat down.
Houseboys and maids would bring out the food in bowls to be passed around each table. If the housemother needed something, she would ring the bell beside her place. Then the houseboys and maids would clear the table, and, as we waited for dessert to be served, we'd sing sorority songs in 3-part harmony.
Now, the kitchens are set up with a buffet. Disposable plates and cups are used. If someone wants more, they go get it at the buffet line.
By the time the first of the line finishes, the last of the line is just coming through, so everyone can be seated to eat. You can even take your plate out on the patio or balcony.
Alas, nobody sings anymore.