'MASS INVIS'

This spell causes all creatures touched to vanish from sight and be
undetectable by normal vision or even infravision. Of course, the invisible
creatures are not magically silenced, and certain other conditions can render
the creature detectable. Even allies cannot see the invisible creatures or
their gear, unless those allies can normally see invisible things or employ
magic to do so. This spell remains in effect until it is magically broken or
dispelled. Thus, the invisible beings can open doors, talk, eat, climb
stairs, etc., but if they attacks, they immediately becomes visible,
although the invisibility enables his to attack first.
 
See also: Invis, Detect Invis.