Being a user on Voice and being a member of an individual site are independent from one another.

So it should be the case that the person in question can still log into Voice just fine (if they have forgotten their password the system can email them a new one).

Independently of that, since you know their email address, you should be able to invite them to be a member of your site from the Members page. They will receive an email with a link in it to confirm that they want to join your site.

Thanks

Joe - Voice Admin