I think I was not entirely clear: we want to do a LIVE session with remote participants, rather than an asynchronous remote session.
Ideally, we'd all work on the same diagram, viewing and modifying simultaneously. To discuss, we'd use a conference call.
It's not so straightforward to find a program that will support this live editing/viewing by multiple users. :)
I don't know about any such service/app; i am wondering won't doing
remote be defeating the goal? you organize and re-organize, then you
group and re-group. there will be voices, comments, opinions in
between. can it be captured remotely?