Role management is handled through the projects endpoints. The projects model contains a users object, which contains three lists: members, spectators, and managers, each lists of usernames. An admin or project manager may set these at any time, adding to or removing from any of the lists. A project must always have at least one manager, however. Attempting to remove all managers from a project will return an error.
This doesn't take into account site managers, and the part about requiring a manager is no longer true.
This doesn't take into account site managers, and the part about requiring a manager is no longer true.