How to book a rehearsal dinner?

Do you want a private room? I agree that just making a reservation for 30 won't likely work. I would suggest looking for restaurants with private or semi-private rooms so you can hear each other and have special wait staff. You might also get a smaller menu of choices or do a buffet. I definitely wouldn't treat it like just going out for a normal dinner with 30 people.

Some restaurants have banquet rooms specifically for large groups. Call around and find the best place!

It depends how you want to do it. Some places have banquet rooms and specific party menus. Other places let you just pay the bill for everyone.

I called the hotel where we have rooms booked, and they are letting us have a banquet room and are coordinating catering for us. Was really easy!

I've seen people rent a private room at a restaurant, reserve an entire restaurant (small restaurant that fits like 30 people), use a smaller room at their wedding venue, and lots of country clubs!

I think looking for a restaurant with a private room is the best bet! Depending on the restaurant they might have fees for reserving the room, or a food and beverage minimum, or set packages for groups of that size.

There are also some venues and restaurants that list themselves as Rehearsal Dinner Locations on WW:

https://www.weddingwire.com/shared/search?utf8=%E2%9C%93&l=y&cid=25&category=Rehearsal+Dinner+Location&geo=Beverly%2C+MA&commit=Find+Vendors

We sent inquiries to the restaurant we wanted, and then booked a private room.