On Compose, Accounts are the billable entities. An account can have one or more users associated with it. These users can create personal access tokens to authorize the use of the API on that user's behalf. The tokens have the same privileges as the user. Databases created by an authorized user are billed to the account.
The web interface for Compose allows owners to manage user privileges, and by extension, the privileges of personal access tokens created by those users. See Access Control in the Compose Help.