I think it would be best to work on this in three parts. The first part would be to craft the page that has the HTMLEditor on it, this page will also have the Universal Email server behavior. You should get this page working first so that you can enter a message and and submit it to have UE send out an email.
The next thing to do would be to modify your results page to add in the checkboxes. You would need to have a form wrapped around your results listing. You would then add in the checkbox and set it's value to the id column from your results recordset. The checkboxes should have the same name and  after them so that it passes an array of values to your page with the UE on it.
The final part of this is to add a recordset to select all of the users to the page with UE on it. Once you have the recordset added apply the DataAssist Search server behavior. You will need to set it up so that you are filtering on the id column, it will be a list type of search. When you get it setup you then would update the Universal Email server behavior to send to each entry in the recordset.
This is a high level overview of how you would put it together. You should work on it one part at a time and test along the way to ensure that you get each part working before moving onto the next.