In some colleges, postgraduates are members of both the MCR and JCR: for example, at St John's, where the MCR is known as the Samuel Butler Room or at Peterhouse. Most colleges also have an SCR.
In certain universities, postgraduates participate in both the MCR and JCR.  St John's College has a specific name for its MCR (Samuel Butler Room) while Peterhouse does as well. Most colleges have an SCR.