OK, I may be missing a trick here, but since I started demoing and integrating messenger there hasn't been an option to invite other users to the conversation. I actually thought this wasn't an option but looking around various help files it gets mentioned a few times and is also in screenshots like this one from your site:

There's also references to it in the language files, like this:

<!-- Messenger Invite -->
    <resource name="Invite_subTitle">Select the people you want to add to this conversation, and click OK. (You can select more than one people).</resource>
    <resource name="InvitationSent">An invitation has been sent.</resource>
    <!-- End Messenger Invite -->

So, why can't I see the button? Has this feature been removed from the release?