Application-Layer Multicast (ALM) is considered a viable solution for compensating the lack of global IP Multicast deployment. While early ALM approaches consider only stationary end-systems, their use becomes more and more feasible on smaller devices like laptops and mobiles. Many ALM protocols have been proposed that employ multi-metric optimization strategies to consider e.g. delays, fanout-degrees, bandwidth or even ISP routing. However, none of them explicitly considers shared access medium domains in mobile environments, e.g. in UMTS cells, which are characterized by limited bandwidth that must be shared among many users. To the best of our knowledge, this paper is the first to explore the benefit of explicitly considering the capacity limits of shared access mediums in ALM mechanisms. To this end, we introduce the concept of shared access medium domains that describes joined capacity limitations for subsets of users. We show that common ALM approaches tend to overload such domains, thus limiting the overall scalability. We extend the optimization strategy of one of this approaches to consider capacity limits explicitly to a configurable extent. Extensive simulations show that our approach can reduce overloading of shared access domains significantly while increasing delays only by a tolerable amount.