This depends on your setup (LDAP or simple database?), aslo on how you are creating users and adding them to the roster. We are creating users in the Admin Console and then use Shared groups to populate rosters to the users. In this case clients shows users' Names.

I'm not sure what you call "Default user and group system". If you are adding users via Admin Console then you must specify their Username and Name. Name == Nickname. If you are using Shared groups to automatically push contacts to Spark users (so they dont have to search and add them manually), then it will show their Names in the roster.