Generating multi-threading implementation of embedded software from design models, i.e. implementation synthesis, is a critical stage during embedded software development, where many nonfunctional requirements are implemented and optimized. Currently, almost all research work on implementation synthesis of embedded software assumes that the underlying operating systems use the priority-based fully preemptive scheduling strategy. However, real-world embedded operating systems may support many kinds of scheduling strategies for different requirements. This paper presents an implementation synthesis method under operating systems that support the group-based scheduling model, which extends the application scope of implementation synthesis. The implementation synthesis method involves task grouping, priority assignment for task groups, task generation and priority assignment. Experimental evaluation shows our method can yield multi-threading embedded software effectively while satisfying real-time requirements.