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How do I notify the IRS of a new member to an LLC?
You don't need to notify the IRS of new members, the IRS doesn't care (at this stage). What you do need, if you have a EIN for a single-member LLC, is to request a new EIN since your LLC is now a partnership (a different entity, from IRS perspective). From now on, you'll need to file form 1065 with the IRS in case of business related income, on which you will declare the membership distribution interests on Schedules K-1 for each member.