You can perform a search on your online/offline guildmates.
For instance searching for 'rogue' will show all rogues in the guild.
Typing: 'rogue ironforge' will show all rogues that are in ironforge.
Typing: 'rogue 60+ ironforge' will show all rogues level 60+ in IF.
Typing: '30-' will show all people <= level 30
Typing: '30-70' will show all characters level 30-70.
GuildFilter searches in the class, name, notes, ranknames, officer notes, zone and level fields.
The only known quirck currently is that the character details screen (that you get when selecting a guildmember) closes when you promote/demote that person.