You can't manipulate the solid state with this forward. What you really need is pev_groupinfo. The ent will be rendered just for it's owner.
PHP Code:
#define Mask(%0) ( 1<<(%0 & 31) )
CreateNPC(...) {
//....
//render it just for player with index id
set_pev( iEnt, pev_groupinfo, Mask(id));
//to make it visible for all players use 0 as third argument
//to make it visible for additional player with index ID add him to the bitmask like
//set_pev(iEnt, pev_groupinfo, pev(iEnt, pev_groupinfo ) | Mask( ID ) );
//to remove him from the group use
//set_pev(iEnt, pev_groupinfo, pev(iEnt, pev_groupinfo ) & ~Mask( ID ) )
}