A shared-cost app could record what group members paid and how they agreed to divide expenses. The record should distinguish an estimated budget from actual spending and a suggested balance from a confirmed settlement.
Splits may vary by item because people join for different parts of a trip. Corrections need a visible history, and members should be able to query an entry. Payment links, if added, require a suitable service and explicit approval.
Start with a sample trip involving unequal participation. Test a refunded booking and someone leaving early. A useful first release can maintain an understandable shared ledger without holding money or implying that every recorded amount is accepted simply because another group member entered it.
Related reading: Purchase Order and Supplier Management Software.