BOERUM HILL - Merchants in Boerum Hill have joined a borough-wide Facebook group to better alert fellow store owners after a string of robberies in the area. Members of the group, called the Brooklyn Merchants Against Crime (B-MAC), can immediately warn each other by sending images or notes warning of suspicious behavior. According to B-MAC organizers, 10 merchants have signed up and 15 are in the process of sending in their paperwork.