I agree that a member search function is needed.
As far as the opt in/opt out debate, I would think that would be a group's decision.
The way I would handle it is to make the Owner/Administrator choose between an empty radio button choice. (No default selected)
If no choice was made, when saving the settings, the default would be opt out.
The reason I say empty would, I think, subconsciously make the Owner/Administrator at least think about it.
Each choice should be adequately defined since not everyone is a nerd.