The problem is that I never really got them to click an opt in for a newsletter or anything (but it is part of the terms of service).
I definitely don't want any issues caused by emailing them (especially with my hosting company). I don't want to use a third party emailer such as Mailchimp so will be sending my own emails from the hosting server (even more reason to stay perfectly "legal").
I will definitely have opt-out links in the emails so, at worst, I will be sending just one email to everyone with a website where they could immediately and automatically remove themselves.
So my question is whether I am being overly cautious. Between the fact they all registered and added their link info and the email has an opt-out am I totally within the guidelines?