There are several usernames that match the (short) name of the group so that the name of the group is not listed when I type it into the "Change Reviewers" field. Instead, I just see all of the matching usernames (unblocking and blocking versions) in the typeahead results box.
Use the "browse mode" dialog to see more results. This is, as far as I know, the only option when there are more results than can be displayed within an inline typeahead.
tldr; To open the browse dialog, simply click the magnifying glass icon at the end of the input field.
This is covered, albeit briefly, in Phabricator's Search Documentation