Hi Andy,
Please navigate to the Cost Management + Billing -- Billing profiles blade using link below:
https://portal.azure.com/#view/Microsoft_Azure_GTM/ModernBillingMenuBlade/~/BillingProfiles
On right, click on the billing profile you would like to add role assignment to. Next click Access control (IAM) blade, and then click Add. Select Invoice manager and then search for and Add user account.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP