Is there a way to make the groups within iWiccle private and by invitation only?

Coded into 1.20 already. =)

awesome - and presumably Wiccle as well?

Yes, naturally. iWiccle 1.20 currently exists inside Wiccle 1.0. =)

Sounds more and more like Wiccle is the solution for my site. Can't wait

Well, at least the backbone solution. Unless you have a module that sends over paying members too... lol