I'd rather have it opt-out :P
If it's opt-in the initial email will only be seen by those who are active on INGO to begin with, and those who are probably already involved and know whats going in in the state.
In fact, most members are probably opted in to receive notifications from the forum anyhow, on most forums that is the default setting when registering unless a member specifically unchecks the box that says something like "Receive news and alerts from admins".
So in a way, a ton of people have likely already opted-in, they may just not know it because they don't receive emails. A clear link and description on how to opt-out from any future messages would be better, I believe. But that's just my $0.02.
By "opt-in", I'm referring to a mechanism by which the option is presented at registration, and can be configured via the INGO UserCP. If an existing option can be used, that would be great.