To make changes to existing users in your organization (or add new ones), first click on the user icon in the top right corner and then click on Manage Users.
You will then see a list of all existing users part of your organization.
When adding or editing users, you can only add or manage permissions for which you have access. For example, if you have a role of Manager, you can add or edit permissions of Users or other Managers, but you cannot edit Admins or give anyone Admin privileges.