One owner or several isn't about size. For tax, the IRS ignores a single-member LLC and treats the owner as the taxpayer, and the company files one information return a year. An LLC with two or more members is a partnership with its own tax return, and its operating agreement has to settle in advance what happens when the owners disagree. Both are common. Here's what changes.
Side by side
What the operating agreement has to settle
With one owner, the agreement records that there's a single owner, that the company is separate from them, and how it would be closed. With two or more, it has to settle the things co-owners otherwise argue about: the ownership split, who manages and signs, which decisions need a majority and which need everyone, how and when profit is paid out, whether a member can sell their share and to whom, and what happens if a member leaves, dies or stops contributing. Make sure yours covers each of these. Standard and Premium record the ownership split, so the agreement and the tax return match.
What changes at the bank
Every owner of 25% or more is identified with a passport and proof of address. Banks and payment processors ask which members can sign, and the operating agreement answers that. A multi-member LLC gives US clients a W-9, as a US partnership. A single-member LLC's foreign owner gives a W-8BEN. Marketplace tax interviews follow the same rule.
Adding a member later
When a single-member LLC takes on a second member, it becomes a partnership for federal tax from that date. The operating agreement is amended, the new member is identified to the bank, and the tax year is split: Form 5472 with a pro forma Form 1120 for the part before the change, and a partnership return for the part after. In Wyoming and Delaware nothing is filed with the state, since members aren't on the articles. Tell your accountant the date when it happens, because a split year is routine if they know in advance and awkward if they find out in April.
Adding someone for the wrong reason
Founders sometimes add a co-owner for a reason other than ownership: a spouse for inheritance, a friend to reassure a bank, an investor on a promise. Each one turns the company into a partnership, with everything above, and a co-owner is a co-owner whatever the reason. If the point is inheritance, the operating agreement can say what happens when you die without adding anyone now. If the point is investment, an investor in an LLC is a member with the rights the agreement gives them, and that's a conversation for a lawyer.