Even with the assistance of scalable video coding (SVC) and adaptive modulation and coding (AMC), the Internet service providers (ISP) are still challenged by videos' surging traffic, compromising the quality of service (QoS) of end-users. To further promote the efficiency of real-time video distribution, we seek aids from the device-to-device (D2D) content distribution technique, where the user equipment (UEs) helps relay content to its nearby neighbours, reducing the transmission time of less popular video layers. To accomplish this, We introduce a new Dynamic Coalition Algorithm (DCA) which allocates spectral resources among coalitions based on their demands. The DCA consists of warm-up and update modules to handle the mobility of UEs during the transmission of real-time video. Multiple experiments show that our algorithm achieves good performance with lowered computational complexity, accelerated convergence, enhanced experience of services and robustness when broadcasting long videos.