It's the person the authorities can point to as having corporate authority to make things happen or take responsibility for what does happen, so they'd better make sure they have the power in the first place, or at least get paid enough for taking the can! They would therefore need to be directors, at least.
If they're accountable for engineering, they need to be up to speed with the new EASA stuff, and be able to demonstrate that they did the paperwork themselves and have read it. The same goes for JAR OPS
Phil