You need to enable: Cloud User Accounts API. After that you can use the Users:list API call to obtain the list of users of a project.
https://cloud.google.com/compute/docs/access/user-accounts/api/latest/users/list
For a BigQuery dataset you can use the API calls under Dataset:
https://cloud.google.com/bigquery/docs/reference/v2/datasets#resource
access[] list [Optional] An array of objects that define dataset access for one or more entities. You can set this property when inserting or updating a dataset in order to control who is allowed to access the data. If unspecified at dataset creation time, BigQuery adds default dataset access for the following entities:
access.specialGroup: projectReaders; access.role: READER;
access.specialGroup: projectWriters; access.role: WRITER;
access.specialGroup: projectOwners; access.role: OWNER;
access.userByEmail: [dataset creator email]; access.role: OWNER;