This addon detects (by mouseovering/targeting) which daily visitor is in your garrison, this data is then transmitted to other guild members with the addon installed, who will find the visitors icon in the GuildRoster Frame next to the class icon. Mouseovering the icon gives additional information about the visitor.
In the upper-right corner of your GuildRoster Frame there is a button that toggles the visibility of the visitor icons.
In the blizzard Interface/Addons options you can enable a "stealth mode" that prevents your guildmates from seeing your daily visitor.