This paper presents an architectural model for contextual-based delegation access control for pervasive computing, particularly access control mechanisms for ad hoc coalition scenarios. Coalition access control encompasses control mechanisms dealing with access between multiple heterogeneous entities from different security domains. For pervasive computing these coalitions are formed in an ad hoc manner and need to be sensitive to the context of the participating entities. An architecture model called the SIP Session-based Coalition Access Control Architecture (SCACA) is presented that extends the SIP signaling model in order to support long distance delegation access control. The result is an architecture that supports spontaneous ad hoc coalitions for a pervasive computing environment.