Winston Starts, a nonprofit group that aims to accelerate the growth of startup businesses, plans to open space in February to 12 tenants on the fourth and fifth floors of the 18-story GMAC Insurance tower in downtown Winston-Salem. The startup tenants will be able to stay in the leased space for up to 30 months. The goal is to have up to 50 tenants at one time, based on one to three employees per company. Tenants will pay $125 per employee per month on the fourth floor and $200 per employee on the fifth floor. For some tenants, a percentage of their rent money will be set aside in an account for future use.