"You can target one additional creature for each additional mana expended.
Continued Invisibility. If you expend 1 additional mana the duration changes to 1 hour/mana."
It can be interpreted as if the additional mana expended to cast Continued Invisibility also increases the number of targets, which I guess isn't the intention? If it is, then it's fine.
The same text is used throughout every spell that increases the number of targets, such as Levitate.