To give the user full access to the hosting account, to be able to make amendments, you need to add the Account Administrator role under:
Account > Admins > (User) > Manage Advanced Settings > Add Element
This will grant users access to the entire account; it is not possible to limit access.
How To Create A New User
You can create a new user in cp.blacknight.com under:
Account > Admins > Add New Admin
When creating the user you will be asked to assign a Billing role - there is only one Billing role current i.e. Full Access. It is not possible to set up new Billing roles, where this role will give the user full access to the billing side of your account.
However note that unless you add the Account Administrator Hosting role to this user, they will not be listed as a full admin - for Technical Support purposes, and the user will not be able to make changes to the hosting elements of the account.