Per-case access lets you give each user None, Read (view a case but not change it), or Write (view and make changes) access to any given case. This is a premium feature — on plans that don't include it, every user has full access to all cases.
There are two places to set it.
From a user's profile
Open Account Settings → Organization → Users, open the user, and scroll to Case access. Each case has a None / Read / Write control; use the search box to find a case, use Grant all cases to set them in bulk, or click Show all to reveal cases the user can't currently see.
From a case
Open Account Settings → Organization → Cases, open the case, and use the Assigned Users card. It lists every firm user with the same None / Read / Write control and a search box.
Changes save as you make them. Note that the Enron sample case is always available to every user for training, regardless of these settings.