Solutions
The company is the member. Its people come and go.
In a trade association the member is a business, and what it joined for is a place at the table for everyone it employs. People arrive and move on all year, and each company keeps its own list current without anyone on your side being told. A membership that reaches a whole staff is one that has reasons to stay.
Bill the company and seat the people
Organizational membership with seats, hierarchies, and a billing contact who is not always the primary contact.
Keep the history when people leave
People leave companies. The company keeps its membership, its spend and its event history.