If your company was formed as an LLC, you would only have an operating agreement, not a shareholders agreement, which is a document only used with a corporation. If in fact you have an operating agreement with more than one member (it sounds as if you do), without provisions indicating the process to add additional members, I am not aware of any "sample" or form for you to use. Because these are complex documents, you really should have both an attorney and accountant advise you on the issues and process for amending your operating agreement appropriately.
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The Legal Considerations of Starting an Online Business