You can no longer get invites...Grand Central has closed the invitations. I don't know when or if they'll open it again for new users.

I have since done a bunch of other research on VoIP. I came across the MagicJack. It's $40 and that includes 1 year of unlimited calling service to the US and Canada, and $20 per year thereafter. I've been able to hack my MagicJack credentials so that I can use my own SIP box, softphone, etc. This could be an alternative for you in case you can't "buy" an existing Grand Central account (perhaps in ebay???).