To the OP... your original post and all the posts are very vague and it seems the people who responded are trying really hard to pull information from you...
The 50-200 which you have is a telephoto lens and in order to take group shots, you will have to be pretty far out, even if the lens is at 55mm.
I would suggest picking up a 18-55mm lens which happens to be quite inexpensive and and will give you most usable range for most situations (can get a used one for about 100$) or a 18-200 which will give you the most usable length on a D90 from wide to telephoto in a flash without switching lens.
After you practice and shoot enough group pictures, I suggest going back and looking at all the pictures to see what focal length you happen to use the most, and upgrade in that field with a faster lens and better glass.
Ask yourself whats missing from your shots and what perhaps ask on how to improve by posting a picture or two asking for cc. Obviously the more expensive lens tend to be better and improve he picture in a few ways, but it will not change your eye; only practice will do that.
Also I found out that libraries carry quite a few books on photography and it really doesn't hurt brushing up on some. This is very inexpensive and it will give you examples and advice which is probably the best way to learn next to taking photo classes.
Now to answer your question; there is no best lens to use because in this case, there are too many factors involved depending on price of the lens and effect which the shooter is looking for.
For example some photographers love using primes for most of their work meanwhile some like using mid range zoom lens or wide angle lens or telephoto lens.
When you ask a question like this, you need to answer a number of things:
- what you're using in terms of equipment (Camera body/Lens/Filters/Flash)
- shooting conditions (dark/light, room/outdoors, sun/shade/cloudy)
- what effect you're looking for
Also you mentioned shooting groups outside, however you never stated the conditions. For example if its sunny the pictures will turn out flat and burned due to the sun and you will require specific filters (
polarizer is a good way to start then
you can look into a ND Filter ). If its dark and gloomy you will most likely require a flash.
From experience, I can tell you that I've taken two classes of photography in HS, owned a film camera since I was 17-18 and 15 years later I'm still learning and upgrading in this field. Its a hobby which keeps on giving by keeping me busy and I love it; so don't take it personally when you're asked specific questions. Some of the users have 20-30+ years of experience with tens of thousands invested in gear, courses and time put into photography, so all their questions tend to be pretty legit... at least 99% of the time.