Get UserCollection for a Project using Bentley Web Services Gateway

I am working on a web app that needs to get a list of users (user collection) that is assigned to a project.  I can query using the WSG endpoint and get a list of every user collection, and also get a list of every project.  However what I am missing is how to query for the relationship between a project and user collection.

Is there the a way to get the user collection for a particular project via the WSG? 

Thanks