Today I had a teacher drop me a line needing a refresher on how to use Messenger on Infinite Campus' Campus Instruction. It may have taken me most of the afternoon to whip this together, but here is a screencast to walk you through the three options. There is no audio with the video to make it easy to watch (and not listen to me yap). I will add in the links mentioned at the beginning of the video as time allows. In the meantime, feel free to fast forward through to the messenger type you'd like to review.
I had a teacher ask me if there was a way to hide or remove students who are no longer in your class. The answer is maybe. It depends on if they are officially dropped from your class and appearing as red in your roster and gradebook. If not, then no, you cannot remove them until they have been unenrolled from the class. If the student is showing up as red in your gradebook though, you can hide them so you don't have to worry about them. Here is a screenshot to help you do it: