If one person in a campaign has a master subscription, then the books owned by all the players in the campaign are shared. (You can turn it off for Curse, so the players can't read it, but still have access to the character options.) This means that, if you wanted, you could split the costs among the group. (You buy the subscription and Curse, and then other players buy the other books.) Except for the subscription, it's a lot like sharing the group's physical books around the table.

There are some other books you might want for extra player options (Xanathar's, Tasha's), but you can play without them.

Nobody else needs to buy a subscription.

There's a FAQ on on of the other groups with advice on what you want to buy for various possible scenarios: https://www.dndbeyond.com/forums/d-d-beyond-general/general-discussion/13989-a-buyers-guide-for-d-d-beyond

As for whether you need DDB if you're playing in person? The tools are handy; they make a lot of the bookkeeping easier. They make it easier to transition to virtual if circumstances change. You probably don't need them, but you may like them, and that's really you and your players' call.

DO NOT GET THE MM.

All adventures unlock the monsters required. Unless you're doing homebrew adventures or you want the lore included in the MM, it's a waste of money to buy it. If you want the lore, you can get the compendium content. At that point, it may be worth getting the whole book - it's up to you.

To share content, only one person needs the subscription, and anyone in the campaign can use any content that anyone else owns in the same campaign.

 Also note that some character features don't fully work, even for items that appear in the PHB. And no temporary effects, like bonuses due to rage or having Bless or Haste cast on you. Some you can make with homebrew items and equipping them, but in some ways it's not as easy as simply writing a note on your paper sheet.

If you have some of these books in paper form, see how much you can get away with not buying them a second time, and play around with just the free Basic Rules options here before paying for anything.

I'm not sure why Beardsinger says the Master Tier isn't worth the cost.  As you expand and grow your collection of books, having those options available to the Players via the Character Creator is absolutely worth it, as is the Campaigns curation.  Master vs Hero is also just a $2 per month difference.  So long as the Player's are also using PC's, Laptops, Tablets, or whatever, making that digital access a shared thing is absolutely worth it IMO.  (small thing, but if you try out DDB and like it with a month's testing, you can go annual for a savings, if you want).

Edit: The only caveat I have is probably a paranoid one on my part, and that's just the concern that with the advent of "D&Done" my books licensing or whatever will expire, or that the Character Creator and other Tools may be retired for 5e, things like that.  I can't make my recommendation without that, as it's not like DDB Staff has truly addressed the concern of how DDB will treat 5d going forward with the release of the next edition.

I wouldn’t necessarily purchase the entire PHB if you don’t need to, as you can purchase individual player options (races, subclasses, backgrounds, feats) á la carte and probably get away with spending less money total if you only purchase what you need for your players. You could also skip the DMG and only purchase the magic items from it, and skip the MM as the adventure will automatically unlock the monsters in that adventure. CoS will probably take you about a year (or more) to complete, so I would recommend getting your subscription as an annual purchase as it will save you money over 12 monthly subs.