All the Broadway ones are open to everyone. I've been to the Something Rotten last year, and my friend when to Annaleigh Ashford's. They don't always share the invites though. I'm still bitter that I didn't know Kristin Chenoweth' was happening, and it was even on a Sunday afternoon, so I could have gone without having to take time off work or anything. Grrr.
But I agree, these are awesome, and I have in fact watched the videos of many SAG conversations online, Broadway and otherwise. I love how in-depth some of them are. Also, for the Broadway ones, I think they also release an edited short version first, and then the full-length one later!