Linking multiple records from a different table to an automation

I’ll do my best to explain this as well as I can.

I have a table with users and a table with invoices
Depending on how many users are active (linked) on an invoicing record the price changes.
In most scenario’s (99%) all users will need to be on an invoice.

Currently, I have set up a form to create a new invoice (a new record in the invoicing table) that requires a manual select of all the users.

Ideally (for now) I would like to automatically add all user in the user table to the record on the invoicing table (linked). I have been looking into the ‘update record’ automation but I can’t seem to find a way to auto-select linked records in there.

Is there a way this can be done?