Congrats on your engagement! I completely understand your confusion, the timing of save the dates and invitations can be really complicated when there are group booking involved. There are few ways to do it and I think it depends on your TA and where your guest are traveling from. We had a longer engagement than most (17 months) so we tried to spread out information so that guest would have enough time to plan and save money if they had to. So we sent out save the date in August of last year (16 mo prior) with info for our wedding website with the detailed booking info. Our deadline for booking within our group rate was December/January (we extended it for some people) and we got a good number of people put in a deposit.
Some people complained because they felt that a year is too far to commit to something, but this worked for us because we wanted to have an estimate of guest sooner rather than later since we invited about 200 guest and we had no idea what kind of response we would get.
I will be sending out my invitations this June or July with probably a August RSVP deadline to get our final head count. Our guest that decide to come who did not commit before the group booking deadline will have missed out on the group rate, but they knew that a year ago. I suspect that we will get a few extra guest but not a ton more.
This worked for us because our group deal only included hotel since our guest are coming from around the world and there would have been no way to include airfare with our booking deal. Most of our guest travel frequently and are used to making their own travel arrangements, so they did not mind. Hope that's helpful