First, to add members to an LLC you would do this by amending your articles of organization with the Secretary of State and adding the new members. You would also amend your bylaws
/membership agreement which would include rules for removal of members, voting rights, responsibilities of members, percentage of ownership of members, and any other regulations you want that you want to specify.
As far as taxation, when you say it is "currently taxed like an S-corp," this tells me you really are not familiar with an LLC and you perhaps expect this to change, but it will not change. An LLC is a "flow through" entity for the purposes of taxation and all of the members pay taxes on the profits of the LLC based upon their percentage of ownership. The LLC files a corporate tax return, but pays no taxes, and each member is issued a K-1 Form
which details that member's tax liability based upon their percentage of ownership. This does not change and you need to make no notice to the IRS or anyone other than updating the operating agreement with the Secretary of State's office and perhaps updating your annual report if you are going to be changing officers around or adding new ones.
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