The invitation should be creative, promising a special family gathering, and maybe including a list of the people you've invited. Ask people to tell you if they'd be interested in attending, and if they know of other family members who should attend. Remember that it will cost money to run the reunion. You have to pay for mailings and phone calls, feed everybody, and maybe even hire a hall or provide entertainment. Discuss with your parents how you want to handle this.
Many family reunions have some kind of price per person to cover food, drink, and other costs.
Plan events for the party. Most reunion time is spent sitting and talking, but it's nice to have a few events planned. You could have a recitation of the family history, a few speeches from the family elders, or a fun event like a dance contest. Get everything in place as the event draws near. When the reunion is just a few weeks away, send out a one-page reminder, or call relatives who said they might come.