Finding people requires clicking on the people word at the top. Almost all users miss this step
When searching for someone to chat with the default is to search within messages. You have to click on the people word at the top. However this isn't clear from the UI. Most users miss that they can click on the words at the top causing them to think that the user they are looking for doesn't exist. In 6 months I've been called by every new employee about this. Turn them into tabs or some other way to highlight that so that they are clickable.