Note that you can't share content unless you have a master tier subscription. Then, you create a campaign and can share your content with anyone in the campaign, deciding exactly how much to share. You can have up to 5 campaigns going at once, with up to 12 separate accounts linked. So effectively, you can share with up to 60 people at a time. I often come close to maxing this out, since our practice at D&D Club is for me to make a campaign for each group precisely so they can all access DnDBeyond. This is working as intended, and is presumably why WotC supports school clubs.
So that's a great deal for large groups, but not much use to anyone who doesn't want or need a master tier subscription (6 bucks a month).
I will note that, as someone who spent years lending books out and then having to track them down, etc. (sometimes even replacing them), sharing via DDB is infinitely more practical, efficient, and inexpensive.
Edit: you can have up to 5 campaigns going at once that support account sharing. I'm not sure how many campaigns you can have going at once overall; I don't know if there is a limit. I've personally had eight active campaigns going at the same time, five of them supported.