Facebook Group to grow and maintain activity

Hi, i have a facebook group in which people post buying and selling articles. I need somebody to grow that group and maintain activity as administrator. The group has 3 years old and 500 users. This is the link of the group https://www.facebook.com/groups/366392847155159. Please make a written proposition of how do you intent to promote this group in order to bring more members.(Please understand that the price will be negociated). Thanks!