There are undocumented features of the REST API, such as GET /users/3.json?include=memberships, which returns additional detail about the project roles and memberships of a user.

See redmine.org/issues/8745.

Redmine API module's API-consumption functions don't allow calling the REST API in this way. Once these undocumented features are better documented we should extend Redmine API module to allow usage of them.

In the meantime, the data can be accessed for all users like this; $users = redmine_users(array('include' => 'memberships');

Or for one user (user ID 3 in this example) like this:

  $user = rest_api_query('redmine', 'users', 'load', 3, FALSE)->set_parameter('include', 'memberships')->alter()->execute()->debug()->result->user;

(->alter() and ->debug() are not strictly necessary.)