Reminder: your Tax Identification Number is your Social Security number, which is placed in the second box on OnlyFans.
Unless there are only two partners who are spouses, the partnership has to file a separate form 1065 partnership tax return that lists all the income and expenses of the partnership.
They should have that information on record and may be asking for it merely to report on dividends and interest paid to its customers.
If you want to track your income on a monthly basis, I'd just create a Google Sheet.
Yes, you can refuse a request to fill out the W-9 but only if you are suspicious as to why a business has made the request.
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