The 6-8 ID number is the unique number associated with an Athlete Profile on the 6-8 app. To find this number, click on "Athletes" on the menu at the bottom of the homepage and then on the name of the Athlete. The 6-8 ID is the number that appears under their name.
If you already have a user account on the 6-8 app but have not yet created an athlete profile (or multiple athlete profiles), click on "Athletes" and then on the little + in the bottom right corner to build out a profile.
Read about the difference between user accounts and athlete profiles HERE.
If you don't yet have any kind of account on the 6-8 app, please download the 6-8 Sports App from the iTunes (Apple) or Google Play (Android) stores. Once you have the app, you first need to create a user account. Once you do that you can create an Athlete Profile for yourself (or others) within that user account.