MettleCI Workbench uses the same authentication subsystem as all other DataStage clients: The Information Server Services tier. This means you can use your regular DataStage credentials to log in to MettleCI Workbench.
The roles in the services tiers are mapped to roles in Workbench. Users with the DataStage Developer role in the Services tier can log into Workbench and can perform everything other than the administrator-specific tasks. Users with the DataStage Administrator role will also have access to some Administrator-specific tasks in Workbench which are currently limited to the Manage Projects functionality.
Note that MettleCI only considers the Services tier roles for permissions, so project level permissions configurations are effectively ignored.