It sound like what you asking for is not SSRS as such: it's an interactive web page. by this I mean SSRS is output only, no data input.
So you want users to share comments on the
latest report execution? SSRS will render with the latest data by default so what 2 users see on different screens a minute apart will be different. How do you matchreport data and comments?
a specific execution? Look at report snapshots.
the data in any report execution? See my idea below.
My solution for "comments on data"... which assumes annotations are persisted in the database with the report data...
The Report Viewer control in remote mode with an ASP.NET wrapper to deal with user input (annotations) might do the trick. The report would display annotations. In SSRS the screen = the report so this captures one requirement.