When you add a member to your previously single member LLC (which you can do structurally by amending your operating agreement and filing an amended report, if required, with your secretary of state), you cease to be a 'disregarded entity' under the applicable Treasury Regulations.Going forward, you will either be a (a) partnership, by default, and will have to file a partnership income tax return on Form 1065, or (b) a corporation, if you so elect, and will have to file a Form 1120 if you are a C corporation or Form 1120S if you elect to be taxed as an S corporation.There can be other tax issues as well, and these need to be addressed with a business CPA.