def add_to_collaborators(self, collaborator, permission=github.GithubObject.NotSet):
        """
        :calls: `PUT /repos/:owner/:repo/collaborators/:user <http://developer.github.com/v3/repos/collaborators>`_
        :param collaborator: string or :class:`github.NamedUser.NamedUser`
        :param permission: string 'pull', 'push' or 'admin'
        :rtype: None
        """
        assert isinstance(collaborator, github.NamedUser.NamedUser) or isinstance(collaborator, (str, unicode)), collaborator
        assert permission is github.GithubObject.NotSet or isinstance(permission, (str, unicode)), permission

        if isinstance(collaborator, github.NamedUser.NamedUser):
            collaborator = collaborator._identity

        if permission is not github.GithubObject.NotSet:
            put_parameters = {'permission': permission}
        else:
            put_parameters = None

        headers, data = self._requester.requestJsonAndCheck(
            "PUT",
            self.url + "/collaborators/" + collaborator,
            input=put_parameters
        )
        # return an invitation object if there's data returned by the API. If data is empty
        # there's a pending invitation for the given user.
        return github.Invitation.Invitation(self._requester, headers, data, completed=True) if \
            data is not None else None