Ten to fourteen days before the family reunion, you'll want to send a reminder to family members that the reunion is coming up. Otherwise, in their busy schedule, they may not remember.
A follow-up invitation adds importance to the occasion in the minds of family members. It makes them think you really mean business. In fact, the more follow-ups you send--up to a point--the more likely family members are to make a serious commitment to attend the reunion. So if the reunion is a year away or two years away when you first start planning it, you'll benefit from sending out more than one follow-up.
If you're sending just one follow-up message, all you have to do is include the key information about the reunion:
For large reunions that take lots of lead time, interim updates on the progress of the reunion are helpful. Let people know some highlights of the reunion.
A follow-up invitation as you get close to the reunion also lets you ask for a rough head count. You can say something like:
"Would you mind emailing me a rough estimate of how many from your family are likely to attend? That way, I can make sure we have enough chairs, food and so on."
Be sure to include your email address unless the follow-up invitation goes out by email.