The team wallet pays. A team project's API key charges the team wallet with no extra header, and a personal project's key charges your personal wallet.
Create a team project: switch to the team, then choose Projects, then New Project.
Move a personal project: if you administer the team, choose Transfer in the project's menu on Projects. The key stays the same and from then on charges the team wallet.
Balance and limits: with a team key, the $0.50 minimum is checked against the team's balance, so a low team balance returns Insufficient balance (code 97) even if your personal wallet has credit. The concurrency limit uses the team wallet and counts all members' team tasks, and the team's model and spend limits apply (for example
This model is not available for your team).
Details: How do organizations and teams work? and Team API Access.