Currently the regular interface provides username autocompletion / suggestions via jQuery's autocomplete widget, backed by requests to http://bitbucket.org/xhr/users/?term=part_of_a_username
Considering that the API allows you to grant privileges to users it would be nice to have some kind of API endpoint that offers the same functionality as mentionned above. Without it, one has to go back and forth between bitbucket and the client application to verify and/or copy usernames. Not a big issue when it's a limited amount of people, but it does decrease the usability of the privileges-endpoint.
The same thing would probably be usefull for groups as well.