All entries in a single database share the same namespace. Users, groups, information bays, printers, and other entries in the accounts database currently all share one namespace. This means that you cannot have a user with the same name as an information bay, group or other entry in the accounts database.
However, it would be possible to have a host named fredfrog as well as a user named fredfrog as they are stored in separate databases and thus different namespaces.