My husband and I both have 9630's, on the same service, Verison.
He can receive Calendar Invites on his device, but I cannot receive any from him on my device.
He uses two email address, but I only send to his hotmail account.
I have two also, bellsouth and yahoo.
We tried both of my accounts last night but the only thing I would get is a blank email.
I resent the service books this morning and haven't tried it again.
It makes no sense that one can recieve and the other can't and we're on the same service, using the same devices - and live in the same house (okay, that doesn't have anything to do with it I know) ... frustrating.
I wonder if the email addresses/providers you have somehow aren't supported by the invites feature. Out of curiousity, sign up for a free hotmail or gmail account and set it to work with your BB. I don't know that it'll do much but it is an additional step you can take to try troubleshooting the issue...