Recent emergence and popularity of mobile ad hoc networks in a host of current-day applications has instigated a suite of research challenges, primarily in routing and security issues for such networks. The ease and low cost of deployment make this networking paradigm very convenient for group-oriented applications like battlefield missions, business conference, virtual classroom, etc. Such networks are characterized by wireless ``links'', lack of any fixed network infrastructure, rapidly changing topology and mobile hosts. Security for these dynamic ad hoc networks presents many challenges in the area of multicasting for group-oriented tactical missions, operating in a hostile environment. Key-management and secure-routing have been the primary research focus in this area. Source authentication for multicast is also a fundamental problem that needs to be addressed. In this work, we study some of the proposed source authentication schemes for multicast group communication and evaluate one such scheme for a tactical ad hoc set-up. We propose solutions that exploit the hierarchical nature of tactical networks to achieve time synchronization pre-requisites that the proposed schemes have. We define metrics to evaluate the authentication scheme and present simulation results for the authentication scheme evaluated with two different time synchronization techniques. We find that our selected authentication scheme is well suited for a mobile ad hoc network. We show that our solution for time synchronization performs much better than conventional methods suggested for the authentication scheme. We also discuss applications of our overlay architecture for bootstrapping the authentication scheme with reduced communication overhead.