Gmail – How to send mail to group contacts in Gmail without revealing their email ids

GMail is going to stop you from sending that many messages from their servers. They have spam prevention mechanisms

Likely, those messages did not come from Google's severs, as they won't let tens of thousands of messages go out in a day from your account through their servers.

Did you look at the headers people received?

Contacts are just addresses and groups are just collections of contacts. Unless you write a plugin to read user defined fields from the contacts api, I don't believe that you would be able to selectively designation a contact as 'to' or 'cc'.

Otherwise, I would suggest creating two subgroups: one for the 'to's and another for the 'cc's. Remember that contacts can be members of multiple groups. So all of them can still be members of a 'Family' group.