Main Street Employee Ownership Act of 2018 This bill amends the Small Business Act to expand the authority of the Small Business Administration (SBA) to guarantee loans for qualified employee trusts of a small business to purchase the stock of that business. Specifically, the bill allows the guaranteed loan to also cover transactions costs associated with purchasing the stock. In addition, it allows loans to a small business to be guaranteed if the loan proceeds will: (1) be used to provide loans to a qualified employee trust of the small business to purchase the business's stock, and (2) result in the trust owning at least 51% of the business's stock. The SBA must guarantee loans to cooperatives in which employees are eligible for membership for such stock purchases. The SBA shall: (1) provide outreach and educational materials to licensed small business investment companies to increase investment in transitions to employee-owned businesses, and (2) establish a Small Business Employee Ownership and Cooperative Promotion Program to offer technical assistance and training on the transition to employee ownership through cooperatives and qualified employee trusts. The Administrator of the SBA shall coordinate an interagency working group on employee-owned businesses.