Hello Ckooxeachue76
Are you a tourist? Then you probably have got the Swiss Half Fare Card, not the Half Fare Card. The latter is for residents, and only residents do have a customer number.
Holding the Swiss Half Fare Card, you do not need / have a customer number. Simply use your username / password to authenticate with the SBB Mobile app. Then, go to tab Profile and set the discount type to half fare in the passenger details.
You can find the instruction on how to activate the Swiss Half Fare Card in your SBB Mobile app as part of this thread:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card/2
Do the same for anyone in your travel group that also holds the Swiss Half Fare Card.
Does that answer your question?
Kind regards,
Roland