Yeah, it is currently available. Collaborators can be added using the cloud dropdown (only available if you are logged in).
After the other user accepts the invitation, he/she should see the project under "Shared with me" in the open project dialog.
Opening this project should then allow both users to work on the same project! One gotcha is that the users will only see the same code if they are both occupying the same role. As a project can have multiple roles -- or Snap-like subprojects -- they have to make sure they are not occupying different clients as then nothing will be synchronized (and it shouldn't be).
From the "Room" tab, you should see two usernames listed under the currently occupied role as shown below:
I hope this helps! If you have any questions, feel free to ask! (Sorry for the late response)