I think you are using Odoo 12 or earlier, The account_id field in invoice line by default is just shown for the users who has "Show Full Accounting Features" group only. So I assume your accountant user has assigned to  "Show Full Accounting Features" group and you can assign the billing users to " "Show Full Accounting Features" group. If you don't want to assign them to this group then you need to inherit the  invoice_form view and change the groups attribute of the field account_id and add the XML ID for the groups you need to show this field.

The below inheritance will show the account_id field to Billing group and  Show Full Accounting Features group.   

<record model="ir.ui.view" id="account_invoice_form_inherit">
    <field name="name">Account Invoice</field>
    <field name="model">account.invoice</field>
    <field name="inherit_id" ref="account.invoice_form"/>
    <field name="arch" type="xml">
        <data>
            <xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='account_id']" position="replace">
                <attribute name="groups">account.group_account_user,account.group_account_invoice</attribute>
            </xpath>
        </data>
    </field>
</record>