In this paper, we present the design of Social BitTorrent Agent (SBTA). Through SBTA, BitTorrent users can be organized into a friend group in which as one member start downloading files, it will multicast the corresponding torrent files to other members who are also interested. In this manner, torrent files can be quickly spread to a large number of BitTorrent clients, and BitTorrent Swarm will form swiftly too. Compared with the traditional torrent file searching approach, a user can save the time and effort of searching and downloading torrent files. In addition, SBTA also provides functionalities of cooperation with social web sites to quickly prolong the user's friend list. On the premise that friends can trust and help each other, we believe that SBTA can reduce the situation of free-riding. Finally, we present real-word experimental results that illustrate the effectiveness of SBTA.