Keep the application and fee confirmations, the USCIS instructions received, everything submitted through the official portal, records of the gift when instructed, and any correspondence about maintenance or transfer. Add the internal approval that authorised the spending. One file per sponsored person, kept current, makes a later question about a specific case answerable.
Decide who owns the file before the first case
Sponsorships outlast the people who arrange them. Name a custodian, a backup, and a review interval, and keep the records where a departing manager's mailbox is not the only copy. Note which documents belong to the employee personally and how they receive their own set.
When a second or third case begins, the existing file should show what was actually done last time rather than what someone remembers doing. Decide who owns the file before the first case, because sponsorships outlast the people who arrange them. Keep the application and fee confirmations, the instructions received, everything submitted through the official portal, records of the gift when instructed, and any correspondence about maintenance or transfer, together with the internal approval that authorised the spending.
One file per sponsored person, kept current, makes a later question answerable in minutes, and the next case should be able to learn from this one.