In the context of open distributed systems, subject to continuous addition of service offers and constant evolution, suitable (re)use of software components remains difficult to achieve. Improved support for dynamic reuse and accommodation of services is required to cope with the diversity of object implementations and specifications.In this paper, we introduce a programming abstraction called the service group. A service group represents a mediator between weakly coupled distributed objects. It allows the transparency of access to an aggregation of instances of different types related to the same domain of use. It unifies the interface specification of the same service provided by different objects. This is achieved thanks to a type conformance relation between similar services.This approach has several advantages for client objects ranging from late binding improvement to service availability increase. Another advantage lies in the fact that this abstraction enables the composition of existing services and the definition of coordination patterns providing new generic services.