Adding a new LLC member is relatively simple and generally involves having a resolution to bring in the new member and then issuing the membership interest. Simple—however—is the last thing you two should be focused on. As an initial matter, issuing an interest may be a taxable event to you. There may be ways to mitigate this depending on the stage of the business and the LLC interest issued.
The most important matter that should be discussed and completed is a written operating agreement. The failure to communicate and discuss your mutual roles and rights may lead to disagreement that may destroy this business. Ideally you two should discuss your expectations, roles, duties and exit plans and then have that discussion embodied in a clear agreement. This agreement will set the duties, voting requirements and procedures, profit and loss allocations, dispute resolution, dissolution, among others. Without this written agreement you are left to possibly some vague oral agreement, state statutes and the courts. In short, it could be an expensive and time-consuming mess if there are problems down the road.
You should speak with a local attorney regarding your issue. Contact people you know and trust for referrals. If you have no referrals, contact your local bar association for their referral program. Good luck.
DISCLAIMER—This answer is for informational purposes only and discusses general legal principles, trends, and considerations and is not intended as specific legal advice regarding your question. This answer does not establish an attorney client relationship. For legal advice, you should retain legal counsel in your state for advice regarding your specific circumstances.
Adding a person as an owner to a company is not difficult. However, you would consider how decisions will be made, who will be in charge, etc if you will own 50% of a business. In this type of arrangement, deadlock is a realistic possibility, so a well-drawn Operating Agreement should be put together to clarify these issues before they become a problem.