Giving Retirement Options to Workers Act of 2018 or the GROW Act This bill amends the Employee Retirement Income Security Act of 1974 (ERISA) and the Internal Revenue Code to authorize a new composite multiemployer pension plan, which combines features of defined benefit and defined contribution pension plans. Under a composite plan, employer contributions are set at a fixed rate. Benefits are based on a formula, are paid to participants in the form of life annuities (except for benefits that may be immediately distributed from certain plans with a low value), and may be reduced based on the plan's funded status. The plan sponsor must take corrective actions through a realignment program whenever the plan's projected funded ratio is below 120% for the plan year. The realignment program may include measures such as benefit reductions or proposed contribution increases. A composite plan is not covered by the Pension Benefit Guaranty Corporation (PBGC) or required to pay PBGC premiums. The plan sponsors are also not subject to liability for withdrawing from the plan.