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 "Athlete Profiles" within your 6-8 app account 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 "Athlete Profiles" and then on the little + in the bottom right corner to add a new athlete (make sure to choose 'New' and not 'Existing').
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.