This paper presents an approach for managing access to services over presence-based communications based on the context of the situation. This paper addresses two technical challenges. The first is to form a communication group based on an association such as physical location. The second is to dynamically create and enforce a security policy by using service description and the user association. We validate this approach by implementing a prototype based on the IETF SIP/SIMPLE presence protocol for observing a user?s context to form an association group. In addition the security policy is dynamically created and enforced on a packet filtering firewall for blocking unauthorized access.