Speedlight photography suggestions?

My friend, who knows I'm an amateur photographer has asked me to do some photography for his sister's 21st birthday party. The idea is to have a 'photo booth' type setup where there will be props available for people to come and have their photos taken. The location, I have been told (not likely to be negotiable), is going to be in their front yard in the evening (probably from late sunset to after dark), the background I'm pretty sure is some trees or maybe their house (I will try and setup with the trees in the background). I have never done real flash photography before as I don't have a speed light but I will be ordering an SB-700 in the next couple of days. I will be shooting with a Nikon D7000 and 50mm F/1.8G (only lens I have). I'm aware that most of the time some form of light modification is necessary so I have spent quite a bit of time looking on the internet for suggestions of how I could set this up but I can't find any examples of night portraits outside. I'm not sure why this is the case but I would really appreciate some ideas for how I should go about off camera flash. On the night I will probably do some mixed off camera and on camera flash depending on the type of photo but my physical location is unlikely to change (I have been asked to be in the one locations). I guess one of my biggest questions is in terms of hiring extra equipment: softbox or shoot-through brolly? I'm a complete novice at flash photography, remember, but I understand many of the principles regarding size of light source and distance from subject increasing softness of light. How are these factors affected when it is after dark, outside and the SB700 is my most prominent source. Also, I will only be able to use this one speed light. And finally, in terms of composition (in relation to how close any modifiers can be to my subject(s) - there will be individuals and small groups or pairs): because I have a DX camera body there's a crop factor of 1.5x so the 35mm equivalent focal length of my lens is 85mm. I have my own tripod which is fine for my camera but I assume I will need a 6-9' light stand for the speed light to get a nice 45 degree angle? Apologies for the long explanation but I really need help for my situation. Responses from professionals would be highly valued and I would appreciate it if no one gives me links to websites unless they directly relate to using off camera flash for night portraits.

There are plenty of tutorials on setting up portrait with only one light source, the setting being outside at night doesn't make a difference. The basics on lighting the subject remain the same, except you have no sun as a rimlight.
Use the flash as a key and maybe foamcore as a fill?
I shouldn't give you advice on setting up the flash as I have never used it, but if you are lucky enough to get an answer from Deep Blue you should be sure to take it in.
In the meantime you can get out there with a baby doll sat in a chair and experiment. Be prepared to get weird looks.

The main issue you will have is distance. A speedlight will only go so far in that regard. Using a light modifier - diffuser, etc. Will not enhance the distance (and in fact, may reduce the distance).

The primary purpose for a diffuser is to soften the light so you do not have harsh shadows. This means the light will look more natural and not from a speedlight.

You should not have any issues with individual portraits, but group portraits you may find lacking in lighting as you will have to backup a bit. In that regard, you may wish to use your speedlight remotely, which can be done with your camera.

Here is a series of tutorials you may find informative: http://www.adorama.com/.../AdoramaTV

Several of the tutorials discuss lighting in a pitch-black studio inside, so some of those techniques should be applicable to your situation.