/**
* @return used letters in selected group
*/
public Set<String> getUsedLettersInGroup(Group group) {
Set<String> letters = new TreeSet<String>(new LettersComparator(LettersComparator.Z_A_ORDER));
for (Server server : group.getServers()) {
for (SharedFolder folder : server.getSharedFolders()) {
letters.add(folder.getLetter());
}