Full usernames are displayed, but are not searched for when entering a name. This is true both when adding recipients for a message and when @mentioning groups in a topic.
I’m pretty sure this has been reported before, but I can’t seem to find where. I don’t see it in the list for 1.9. Can you add it to the next release? It seems like a small thing, but it drives me absolutely bonkers.
(We have a @digped-cmte, short for “Digital Pedagogy Committee”. I keep typing “pedagogy” and waiting before I remember that the full name of groups is not indexed.)