e-Newsletter: How to resubscribe members

I was experiencing an error which caused people to be unsubscribed. Why do I think that? a) I haven't sent out a newsletter yet and b) everyone who signed up after a certain date was unsubscribed. I think it had something to do with trying to put the sign-up form in a PopUp but I'm not sure.

Anyhow, is there a way to resubscribe people? I tried adding them to another group but that did not work.