You can add athletes through the online Dashboard or through the Dashr App. We suggest you do it through the Dashboard as that is most efficient. Please see the AutoID - Barcode forum post for details on completing this.
Note that you need Roster Spots in order to put athletes on your roster. Note that you do get 10 free spots when you create an account.
See What is Dashr|Sprint and AutoID Technology? for more information on you ability to manage athletes and their data.