Hi.

If you go to Site Pages and Participants, scroll to the bottom and click 'Show All XXXX' (where X is how many users you have), then in the resulting page, click Select All at the bottom and then Choose> Message at the bottom to send all a message, you get a "A required parameter (formaction) was missing" error.

FYI, just in case this makes a difference, I have 2965 users currently.