Huntington Beach Restaurant for 30 people?
I don't know if you are willing to go "HB-adjacent"... but Park Avenue Dining is in Stanton and they have a separate room that can hold 40-75 people. I had lunch there and it was yummy. There have been posts about it as well. I saw the private dining room and it is very, very cute - 1950's diner style. They have a separate menu for private dining (I asked about the banquet menu when I went and I think lunch ran around $20 and dinner $27 + service charge and tip).
Or, you might be able to reserve a bunch of spaces in the main restaurant. Anyway, if your group was willing to go a little further, I thought about this option.
Here is their website: