Update: We chose our cast Feb. 2nd and everyone has been sent invites to the cast version of the callboard. I would say the biggest drawback to the invites is that you can't choose for someone that they receive email notifications of changes to the site; the actor has to sign on and choose 'subscribe to site changes.' I encouraged everyone to do just that, and so far there have been no problems...I must have a responsible group of actors

I post to the callboard before 10pm the night of a rehearsal and it sends the notification of a site update to the actors. This has been the only callboard notification they've been receiving; I haven't been asked to send emails in addition to the notifications.
We had issues with Google Calendar working correctly when attached/inserted onto the Calendar tab of the Google Site. At first I made the calendar private, sent out invites, then attached to the site. I tried every variation I could see to make it work, but most times the actor got a message saying they weren't authorized to view it when trying to view it on the site. Currently I've made the calendar public and reattached it to the site, and so far this is working fine. I'm sure there's something I'm overlooking that will let me make the calendar private while still being able to view it on the site, but this will have to wait until the next production.