I'd say the goal should be to provide a wide range learning tools.

What you're really missing is a fast lens. With the 18-55 and 55-200 you'll really be limited to very 'constructed' shots if you want to shot something that displays good subject isolation - like a classical portrait (subject in focus and background blurred way out). Something like the 50mm f/1.8 or 35mm f/1.8 are both around $200 USD and provide a great learning tool in this. I would lean towards the 50mm just because its more of a portait focal length and gives you that option.

In addition to this, consider getting a set of extension tubes for macro shots, its another option that you'll give your members to play around with. You can also try reversal rings for some extra macro on your 18-55.

One day, when you have the money you may want to look at lens to give your members options to shoot birds and wildlife (lenses that go up in the 300-500mm range)- but I suspect they're out of your current budget. Options start around $1000 USD and go up for quality.

After this, consider stuff like flashes, inexpensive radio triggers, circular polarizers, backdrop stands ,and tripods.

I recommend against a 18-200, unless you know you're going to go out a shoot a mammoth range of things in the same shoot. The quality of the lens is comparability low and you lose a lot of creative control by only having slow lenses.