All account members MUST have an active, pre-existing AT&T prepaid account, to be invited to join the multi-line group. That being said, perhaps they can then be "invited" to join the 25GB plan, without needing to purchase a Walmart phone themselves? As long as the Account Owner has purchased the Walmart phone.
Given that the account owner selects the applicable plan for each member line, and "invites" members to join the multi-line group (on the plan they've been invited to join).
The way AT&T describes it:
Multi-Line Account Owner:
** Starts the Multi-Line account and invites other AT&T PREPAID users (Members) to join.
** Selects an eligible monthly plan for each Member Line.
Multi-Line Account Member:
** Accepts invitation from Account Owner to join Multi-Line Account.