That is the meeting-room ______ they used to have meeting.

此时用where和in which都是正确的。

where引导的定语从句表示地点，which也可以表示地点。区别在于where是副词，前面不能加介词，而which是代词，需要加介词。例如：the house in which I live.而the house where I live.不可以加in。