Expand a multi value field in REST API

I have a Sharepoint list that has a multi value field which stores Person or Group.

In the GET call to the Sharepoint list, I get the following response. Collaborators is the name of the field in discussion.

"Id": 1,"ContentTypeId": "0x0100B86E441274F847A599E46276483F9FBB009692E8772BC86A4F8BB8570C018F79BC","Title": "One Microsoft Way","StartDate": "2015-04-15T06:55:19Z","EndDate": "2015-04-15T06:55:19Z","CollaboratorsId": {"results": [
                    18
                ]
            },"ID": 1,"Modified": "2015-04-15T06:55:26Z","Created": "2015-04-15T06:55:26Z","AuthorId": 1,"EditorId": 1,

I get an array of IDs in the response, but I wish to expand that field to show the Person or Group name. I tried to use $expand=Collaborators&$select=Collaborators/Title. But this does not work.

Any way to expand the values in a multi-value field?