Workspace plans can be adjusted any time by navigating to Workspace Settings > Plans & Billing. From here, you'll select Upgrade or Manage Plan.
The plan you select will become the default for each new member you add to your workspace. For example, if you select the monthly Standard plan ($15/mo) for your workspace, each new Editor or Admin member will be added for an additional $15/mo. All billing is directed toward one contact that can be adjusted by any Admin or Owner under Plans & Billing.