1. Login as a teacher and go to the Participants page for a course like https://qa.moodledemo.net/user/index.php?id=2
2. Select all the participants either by clicking on the checkboxes for all the users or selecting select-all-participants checkbox
3. On the QA server that would select 3 users
4. In "With selected users." dropdown choose "Send a message"
5. See that the popup says "Send message to 4 people" instead of the expected "Send message to 3 people".
6. Also, when I click on submit, I never seem to get the success message. So unsure if messages were actually sent.

The error seems like it is counting the select-all-participants checkbox in the number of users, because if you unselect one user it says "Send message to 2 people" and I do see a success message