Let's say I have an array of objects that look similar to this:
users: [1, 2, 3]
Pluck 'n' flatten will do what you want:
var result = _.flatten(_.pluck(data, 'users'));
As Richard points out, pluck was removed from lodash in version 4.0.0 but still remains in the current version of underscore at time of writing (1.8.3).
Replacing pluck with map works for both lodash and underscore:
var result = _.flatten(_.map(data, 'users'));