I use Arrow. Admittedly it'd be nice if the tool was open sourced so we could continue to develop and improve in it. What issues are your colleagues having importing your exports? I haven't had any issue with the tool doing this.
It is a bit counter-intuitive, but all you have to do is hit 'Export Markup' and paste the shared export in there, then you can edit the model from there.
one that I recently found is yWorks yEd Graph they have both a desktop and online version. It saves .GraphML files they can be used with any other standard modeler that accepts the graphml xml format
Do you update Arrows content from here ?
My colleague is also using Arrow
Tried with two (different) browsers, I cannot copy from first to update second's content
You should be able to move XML between browsers and have them display the same nodes & relationships. Can you see the XML when you hit 'Export Markup'?