A site administrator can add, edit and delete users (logins).
Most of the time, if you are adding a user, it will for someone who is already in OpenCase as an actor - as a volunteer, for example. There is a box right at the top of the form in which to link the user to their record in OpenCase. Start typing the name and it should appear.
If you haven't created the actor yet, you can create the user without one but don't forget to link them later by editing the user record.
By linking them, you let OpenCase know who the login is for, and therefore which cases they are involved in and should have access to.
After saving the new user you will be taken back to the user list page, where you assign a role to the new user. Tick the box to the left of the new user and use the Actions dropdown to add the role. The default roles are described here.
You can change a user's role at any time. Just go to the user list, tick the box next to the person and use the actions dropdown to add or remove roles.