We introduce a method to elicit and to structure using Event B, processes interacting with ad hoc dynamic architecture. A Group Communication System is used as the investigation support. The method takes account of the evolving structure of the interacting processes; it guides the user to structure the abstract system that models his/her requirements. The method also integrates property verification using both theorem proving and model checking. A B specification of a GCS is built using the proposed approach and its stated properties are verified using a B theorem prover and a B model checker.