The paper addresses the problem of client--service interaction in the case of replicated service provision. Existing systems that follow the State Machine approach concentrate on the synchronization of the server replicas and do not consider the problem of client interaction with the server group. Client interaction is analyzed and a number of access protocols are proposed to meet a range of client requirements. The paper demonstrates that protocols for the ``open'' group model---clients external to the group of servers---satisfy the requirements of the State Machine approach, even when replication is transparent to the clients. Experimental performance results indicate that the ``open'' model is clearly desirable when the service is used by a large, dynamic set of clients.