To add a user, simply go to Settings -> Users and select "Add User". Settings are accessed by clicking on the gear icon next to your initials in the top right corner of the web console.
Once a new user is created, they will automatically be emailed with instructions on how to create an account.
Once a user is created, you can assign roles to them.
You can also plug into your existing SSO provider for authentication and even setup automatic user provisioning and de-provisioning by setting up SCIM.