Drupal - REST Services module - Access denied for user anonymous

You need to make the user login who have the permission to view the list of user details using following Service API

http://yourdomain/endpointname/user/login 

with follwing json data as arguments

{
"username":"username of the user who have the permission to view the users list",
"password":"password which matched with username"
}

After the successful login you need to get the CSRF token which will play the key role in all authenticated request. To get the token call the below mentioned API,

http://yourdomain/endpointname/user/token

You will receive a token, then add the token to your postman header for the key named X_CSRF_TOKEN. Then you can start to make the authenticated requests.