Hi there Templar49. While on a Family plan, only the primary line could be eligible for BoostUP. Any Boost customer with a monthly plan who has made at least 12 consecutive on-time payments is eligible for BoostUP. To check your eligibility, sign into My Account. If you are eligible, you will see ‘You are eligible for BoostUP! Installment Billing’ in the My Device section. If you are not eligible the option would not be available.
A customer/account may only have one installment agreement at any given time. Please go to https://www.boostmobile.com/?INTCID=HP:T:4:BoostUP!#!/support/faq/account-management/BoostUP/.