Is everyone in the group going to be on skis? Because Deer Valley and Alta do not allow snowboarding, so even one boarder and the group can't stick together at either spot.
Deer Valley has the most for your group, with really great grooming and fantastic on-mountain food. And it's likely to be less crowded than the others (with the possible exception of Solitude) on Saturdays/holidays/powder days.
Snowbird is definitely steeper and more intense than most anywhere else. But it's also an incredible mountain well worth experiencing if you can. I'd save it for closer to the end of the trip, and see how your novices are doing. If they make it to the point that they're comfortable skiing most groomed blue runs elsewhere, then they should definitely be fine with the Baldy lift at Snowbird, at least. And if the bulk of the group is enjoying groomed black runs at the other resorts, they'll have a great time on Snowbird's groomed blues. Snowbird really shines on sunny days, when the crowds stay away but you have great visibility for the spectacular scenery and 3,000-vertical-foot plunges.
I'm more hesitant than others to recommend Alta for beginners. I think folks are forgetting/ignoring that Cecret, Alta's best beginner pod, is gone. Which means green runs are limited to just a few choices under the busy and somewhat slow Sunnyside triple. Yes, there are easier blue runs to be found under Supreme as well, but I would be more hesitant to commit to Alta unless the whole group is feeling confident on blue runs elsewhere.
Brighton's a lot of fun. Milly has great green, blue, and black choices that let the whole group ride the lift together, and the other high-speed lifts all have easier blue options available. Brighton's grooming also stands out, although they go for a firmer groom (which lasts better into the afternoon/evening) than some others. Expect it to be pretty busy weekends and holidays.
Solitude's terrain for beginners is also pretty limited, but at least they can join you for lunch at the base or the Roundhouse. But intermediates should have a good time. Solitude lacks the parking to get really crowded, which can make it a good choice for a busy day if you're sure to get there before the lots close.
I was in Utah for two unusually warm winters. Weather will probably be fine, but if you do run into the misfortune of a very warm snap (a warm rain/snow system followed by warm sunny days is kind of the worst-case scenario), I advise skipping Solitude and Deer Valley. Much of the terrain you'd be most interested in at each is lower-elevation and sun-exposed. But should be a lot of fun in the midwinter conditions that I hope you encounter!