When using the build user vars plugin in a pipeline the current required usage is:

node {
    wrap([$class: 'BuildUser']) {
        // Do something with env.BUILD_USER* vars
    }
}

The 'node' is required here, because of how 'wrap' works. Because of this it is currently not possible to get the build user information outside the context of a node.

Creating a custom Workflow step would solve this. Example:

def buildUser = buildUserVars()
// Do something with any of its properties:
// - name
// - firstName
// - lastName
// - userId
// - email

Or to keep it closer to how it currently works:

buildUserVars {
  // Do something with env.BUILD_USER* vars
}

See https://issues.jenkins-ci.org/browse/JENKINS-30142 for a similar issue, which has been solved, for a different plugin.