Please see the screen recording below, it should be self-explanatory:
Just tried here and it works just fine. Does the active group have members?
My failure. I can see now there are no members in the group.
But then there is another issue - it is not very clear when the members are actually added to the group. I typed a few @usernames and clicked “Save” and considered it as a done deal. But it turns out it was not.
Also, it still looks buggy when the @ sign remains and zero usernames inserted after I select a particular group name. Maybe some kind of alert would be helpful? I.e. “There are currently zero members in the group named XXX.”
That’s the error (I do the same the first time!)
Type @usarname, click Add and then Save.