So, this is probably the opposite problem that usually comes up. My guild is semi-casual, but we typically have 1 or 2 too many people who sign up as tank or heals. What does your guild do in these situations? Rotate people out? First come, first serve/seniority? Take the best player based on heal meters, etc? Currently for Legion we estimate we'll have 4+ players who would like to play as tanks, and at least 2 extra healers.
What's best for the raid team? What's best for the guild?