Jill,
Blend mode 'Add' is in the third group (the lighten blend modes) - it's listed as Linear Dodge (Add) in my version (not sure if older versions just listed it as Linear Dodge).
It's actually the strongest of the lighten blend modes, being a stronger version of Screen, just as Linear Burn is a stronger version of Multiply.
The modes you mention are the most useful for most purposes, though I also find myself using Color and Luminosity on occasion, as well as Difference if I'm trying to align images accurately within Photoshop.
Eric