[Xenon]

not this hack, it changes the usergrouptable, but it wouldn't be hard:

add a field to user table called canseeinvisible, as you see in the hack for usergroups here

then you'll have to add a yesnocode when you edit users from admin-cp (take a look at user.php i'm sure you can found out how)

and then you have to modify all the lines hellsatan poste which handles invisible users
where you can read "or $bbuserinfo[usergroupid]==6" just add "or $bbuserinfo[canseeinvisible]"