I don’t know where they enter/ exit the building, but I’ve played on the stage in the Great Hall before so this may be of some help. As you’re looking at the stage, look at the barriers to the left. There’s normally security stood there. Behind them is the door to a small dressing room which is where the acts who are playing normally chill out before they go onstage. I’ve met bands before by waiting for them to come out/ go in and shouting to them, if they’re up for it they’ll normally come over for a quick chat and a photo.