Developed exclusively for use with Nikon DX Format digital SLR cameras, this lightweight, compact zoom lens is an excellent match for the new D2H digital SLR camera.
The 17-55mm range exceeds 3x zoom to broaden wide angle to medium telephoto photographic possibilities, and the lens maintains fast f/2.8 aperture throughout to make it practical and easy to use. When mounted on any of the Nikon D2H, D1-series or D100 digital SLR cameras, the picture angle is the equivalent to 25.5 ~ 82.5mm on a 35mm  format camera.
Since you already have the 18-200 as your "general purpose" lens... it has covered most of your shooting range... so now... you can look at fast lenses or ultra wides... here are some suggesstions with budget in mind:
Tokina 12-24 f/4 - Ultra wide for landscapes and taking large groups, etc. [as compared to Nikon's 12-24 f/4] - about $700 new
Tamron 17-50 f/2.8 - Fast lens for that range [as compared to Nikon's 17-55 f/2.8] - about $650 new
Nikon 80-200 f/2.8D - Fast telephoto lens but look for the one-touch AF-D or two-touch AF-D models... which are more affordable now 2nd hand - about $700 (1-touch) / $1000 (2-touch) 2nd hand
Then here are some prime lenses for portraits. The range would depend on what kind of shooting distance you are looking for.
Nikon 35mm f/2D - about $500 new
Nikon 50mm f/1.8D - about $180 new
Nikon 50mm f/1.4D - about $480 new
Nikon 85mm f/1.8D - about $620 new
What you can also do is to look through all the photos you have shot with yoru 18-200mm lens. Look at what range you normally shoot at, e.g. I normally shoot at 35-70mm for portraits, and 17mm for landscape, etc. This would give you an idea what lens would complement the 18-200 which you have.
The bigger the aperture... the more expensive... because of the design, construction, glass, etc. Just on the surface, when you put the 50mm f/1.8 and f/1.4 side by side, you can already see the difference in the built of the lens
Having said that, the 50mm 1.8 is a really good lens... and is a good prime lens to start out with...
Firstly... how big is the group you are taking? How much space do you have in the room/place to take that group photo? Why don't you try out by setting your 18-200 to 50mm and take photos without zooming in or out... and you will get a feel of using prime lenses (our feet will do the zooming).
If you want to get new lenses at a good price... look for "The Oracle" in the buy & sell section. PM him what you are looking for.