Teams and monthly quotas
Group people and give each team its own credit quota.
On Team, Teams, admins make teams with a name, the people in it and optionally a monthly credit quota. What a person spends counts toward their team; when the quota is used up, the team's runs stop until the next month and its people are told once. Each team is also a sharing group: share a flow with the team and everyone in it, now and later, sees it.