In need of affordable multi-family resort near Bay Area

Depends on what you define as affordable...

I'd look for a house that is 4000+ sqft with generous common spaces, to be the "host" house in order to have meals together and house some of the people.  You could have some or all of the meals catered if no one wants to cook for 40+. Otherwise it seems like you can make reservations at larger restaurants for 40+.

Maybe this could be useful, I have not been but dream of going :-) http://www.leonardlakereserve.com/

I would also look at wedding venues, that might give you better results. On that note, I can recommend the Highland Dell Hotel in Monte Rio (by the Russian River). You could rent out the whole hotel and they have a communal hall as well. And then add some airbnbs in the area. 

The Union Hotel in Occidental also has several guest houses https://occidentalguesthouse.com/ and you could do a large dinner at the restaurant. No experience using them, but the houses are gorgeous. 

Depending how far away you’re willing to go, I recommend Oz Farm. Some of it is pretty rustic, but it’s an incredible location and experience. Our friends had a very inexpensive wedding here a few years ago and everyone stayed on the premises for 2 nights. Very affordable for a group of that size! http://www.ozfarm.com/

Another friend threw a 30th birthday several years ago in Healdsburg at a big 13,00 acre ranch with housing accommodations for 39 plus areas for tents for those that like camping. There are 3 properties right near each other on the ranch (main house, guest house and carriage house) on which we're staying, with a total of 14 bedrooms, and 39 bed spots.  Many of the beds are in a room with other beds. They had meals catered and also did some cooking of their own. http://anvil-ranch.com/ I believe it worked out to be about $200/person for the 2 nights all in. Not sure how much of it was subsidized by the birthday boy. 

We've done smaller multi-family trips (15-20) a few times, though typically in multiple houses or condos. The keys have been having larger communal areas (e.g., a pool due to the range of ages, a bigger kitchen) as well as enough private space. The most choices we found were via VRBO in Napa and Sonoma, since there are rentals that cater to wedding parties. With your size group, the house could be the focal point and others would have a range of hotel choices.

For other family trips, Bodega Bay and Angels Camp both had multiple reasonably priced rentals in developments with a shared pool. Each city is a reasonable drive from Oakland/Davis, but there were fewer nearby activities if you're planning more than a long weekend.

We had a 50+ person family reunion (ages 1-90) this summer at Asilomar (near Monterey) and it was pretty good. I don't know the price because my parents covered it, but they're usually pretty price-conscious. It has nice old buildings (many designed by Julia Morgan), so it feels more like a resort than a conference center. Everyone was housed in buildings near each other and we ate together in the dining hall (food just okay, not spectacular, but no cooking or cleanup - yay!). We rented a meeting room for one day, but other days we met up in the fire pit area or the lobby of the main building (which has a great woodsy lodge-like atmosphere) or other community spaces. The beach is right there, the aquarium is nearby, lots of room for kids to run around, board games to borrow, etc.; it felt like a real vacation. They had an on-call shuttle to help older family members get around the property, which was super helpful. We also considered Leonard Lake Reserve (Leonardlakereserve.com), if Ukiah's not too far away. They have 4 houses you can rent as a group and came recommended by Sunset Magazine.

I might checkout Costanoa near Pescadero: https://costanoa.com/. They have a few different options from camping to cabins to a lodge - onsite restaurant and lots of activities for everyone. Really beautiful area too. Not sure if this is your style but worth a look!

What about renting a few houses up at Sea Ranch and designating one of them as the homebase? There are all different sizes of houses at Sea Ranch  and prices for every budget, and you'd all be within a few minutes drive of each other in the worst case. There is a restaurant with a nice lounge overlooking the Pacific Ocean at Sea Ranch proper, and there are also a bunch of restaurants in nearby Gualala that could accommodate a group your size, everything from pretty good take-out pizza, to cute waterside dining at Trinks, to a whimsical fancy dinner and/or cocktails at St. Orres. There is also a fantastic market in Gualala called Surf Market that has pretty darn good prepared food and sandwiches not to mention fresh local seafood to take home and cook. We've stayed several times at a particular 4BR house at Sea Ranch that could work as a home base - beautiful large open cook's kitchen with a livingroom & fireplace, plus a giant separate den, plus outside seating on the deck around the hot tub.  I always use https://www.vacasa.com for Sea Ranch - they manage a lot of rentals there and have an office in Gualala, so if something goes wrong, there is someone local to help you. 

Asilomar Conference Grounds in Pacific Grove (just past Monterey). It is a campus of lodging buildings, conference rooms, a social hall, and a dining hall set just inland from Asilomar State Beach. Sand dunes with a boardwalk separate the grounds from the beach; it's maybe 1/4 mile to the beach. The dining hall serves 3 meals a day. The social hall has a big fireplace, pool tables, and board games. The grounds are gorgeous and you will likely see deer wandering around sometime during your stay. Since it is set up for conferences of hundreds of people, handling 50 will be easy-peasy for them. The only issue is that you may have to schedule around any larger conferences. Many of the lodging buildings have common rooms, often with a fireplace. There are a variety of reservable spaces if you need something more private than the social hall or common areas. Oh, there's an outdoor firepit, too. They have some handicap accessible rooms (I took my mom there when she was 89 and using a wheelchair).

And of course Monterey is beautiful and has a ton of things to do, including the aquarium.

https://www.visitasilomar.com/

I can only think of one spot with interesting architecture, event space, and rooms. Berkeley City Club. 

https://www.berkeleycityclub.com/

You may want to search wedding venues to see if something in that category works. Or you may want to find event spaces and rooms in buildings near one another. Maybe look for a hotel near a college campus or a hotel near a park. 

I haven’t been, but this place looks great:

https://www.barszranch.com/

holds a lot of people and is somewhat reasonable. 

Take a look at Sequoia Lodge, Leona Lodge, Brazilian Room, Heather Farms, and Lafayette Veterans Memorial.
Closer to Davis you might want to look at Ryde Hotel or Delta Diamond Event Center. 
Half Moon Bay has resorts  https://www.visithalfmoonbay.org/meet/venues
Have you looked at any of the wineries.

Hello, 

It may be a bit too far for your needs, but we just stayed at Asilomar in Monterey and it was really lovely. 

I don't know if it would be too far to go, but take a look at Andril Fireplace Cottages.  http://mobile.andrilcottages.com/index.html  They have one and two bedroom cottages, but they also have 2 large houses that can serve as "home base."  We stayed in "Rancho Reposo"  which had 5 bedrooms, some having multiple beds, full kitchen, nice sized front and back yards, and a large living room with a big sectional and fireplace.  Those who wanted more privacy rented cottages.  Bonus is the location: right across the street from Asilomar, so you get beach access and can go to their cafes, but lodging is lower cost.  Better than renting private houses because you have maid service, free continental breakfast in the office, a hot tub, and nice common areas.