Hence, RemoteRetro. An online retro board, that everyone can log into. Or, throw it up on a projector or a screen, and only the remote team members can join in. Or, no remote team members, and use it to keep a record of the sprint's retro items. Up to you.
It allows users to add retro discussion points to four 'quadrants':
Inviting users to the retro is the simplest as well; copy/paste the URL.
All users can update the retro in real time.
To see this in action, please visit RemoteRetro.