Test pre-requisites
Add the Online Users block
Connect simultaneous with other accounts so that you can see something in the block
Use all the major browsers to test this issue
Test steps
Check for consistency in the block styles, CSS, icons, etc...
Go to Home ► Course ► Reports ► Course participation
Make a search so that you have results
Hack user/message.html:46 (right before `if (!empty($error))` and set $error to something.
Now select a bunch of users and "Send a message"
You should see an icon next to the list of users, make sure it appears nicely
Revert your hack and make sure it still look nice
I didn't find a quick way to display this icon without hacking the code