Don't Starve Together requires that server owners specify a cluster token before the server will start and will allow players to join. This token is very easy as long as you own a copy of Don't Starve Together.
- Launch the game and hit "Play" on the first menu.
- Click on the "Account "tab located in the bottom right cornor of the game.
- Click the "Generate server token" button on the popup window.
- Paste this into the Server/Cluster Token field on the panel and save.
You can now start your server.