Single member LLC to multiple-member LLC
The procedure to change your business from a single-member LLC to a multi-member LLC would vary a bit from state to state. Overall, it wouldn't be too difficult of a process.
In a few states, the members of an LLC are listed on the Articles of Organization. To add, change or remove members of the company, as well as the other articles, you would need to file a Certificate of Amendment with the state offices, possibly followed by updating the EIN account to reflect the new ownership structure.
Regardless of a state, an LLC should have an Operating Agreement, where (among other things) members would be listed with various detail, such as percentage of ownership, initial investments, etc. When changing the ownership structure of an LLC you should have the company's Operating Agreement amended, or if it was never drafted then to draft a new one.